I observed something interesting this week...

Way back in the late 1930's, when commercially viable discharge lighting came on the market, the magnetic ballast produced flicker at twice the mains frequency. Over the years, different mechanisms were used to mitigate the effect (lead-lag, distribution over three phases) until the electronic ballasts came on the market in the later years of last century. The high frequency made for virtually flicker free operation.

Now, cue in the advent of LED lights, the cheap hardware store ones have very annoying visible flicker. This week, I paid a visit to the electrical supplier. All their fluorescent troffers have been switched over to LED edge lit panels. I walked in and there was no apparent flicker. When I was waiting for service at the counter, I spotted one of those retro-style fans, that have metal blades and a metal enclosure. It was running and the blades had that slow movement appearance that is usually a tell-tale sign of flicker. I was kind of surprised that the LED panels produced flicker akin to magnetically ballasted fluorescent.

I noticed that too. Even with LED lamps, the cheap ones tended to cheap out and have noticeable flicker while the more expensive ones tend to have additonal components in the driver that at least reduces flicker.

Cost being the most decisive factor, I guess. Now, cost aside, what would prevent say a LED edge lit panel to have a rectifier to run it effectively on DC ?

The problem also is, the only way to eliminate flicker is by using rather large (for the power) electrolytic capacitors and these are rather problematic components mainly from reliability perspective. Not that it wouldn't be possible to make them reliable enough, but it costs money and mainly building space within the ballast cavity (which uses to be extremely small with modern LED products), so much it forces the other components to operate in not that favorable conditions, increasing the cost there (to compensate with "better" components) as well.
Skipping this capacitor allows you to achieve significantly better reliability for the same budget, power and efficacy.

Why the cheepeese are skipping it is clear - as it allows to cut cost significantly even without giving up the reliability of the rest, along with compromising on the quality of the rest it really saves a lot.
But with "quality brands" I see the market rather went into eliminating flicker with mediocre reliability, I'm missing the uncompromised reliability with tolerating some flicker - as to me many applications don't benefit that much from true flicker free operation, but do benefit from the reliability.

Cost being the most decisive factor, I guess. Now, cost aside, what would prevent say a LED edge lit panel to have a rectifier to run it effectively on DC ?

They do effectively run on DC, or on high frequency PWM when dimmable. At the very least the bargain basement LED panels used at my school all run from switching power supplies.

went into a brand new showhome and had to leave quickly as the LED were flickering at 30 HZ and I could not stand being in the place
every light in the place was on and flickering

 "flickering at 30 HZ"
If they are really flickering at 30Hz (or anything else than 2x the main frequency, so 100 or 120Hz depends on where you are), there is something really bad with the lights, anything other than a "double-mains" frequency can not be any intended design feature (such as intentionally not using an electrolytic capacitor, single stage high power factor driver,...).

Very often it is LED string just failing (some bond wire just broke and is thermally cycling - that uses to be in the 10's Hz range).

Or there is something happening in the electrical installation (some arcing on a loose connection in the lighting circuit), or some crazy SCR regulation of some very high load.
Some permanent magnet synchronous motor starters use to do that - as they connect the mains only at times when the rotor happens to be in a correct phase vs the mains to accelerate it, but once it reaches synchronicity with mains it should remain permanently ON, so they should do the chopping only for very brief time to start the motor, after that it should remain permanently ON. Could be when the motor is overloaded, it never reaches synchronicity so the starter never transitions into the permanent ON "run" state and continue bursting.
This concept is quite popular on things like fixed speed blowers or pumps (e.g. dish washer circulation pumps commonly use this concept). It allows to use high efficiency, yet simple permanent magnet AC synchronous motors with way less complexity than a complete VFD, the drawback is the strong subharmonic current draw during startup, which may cause some lights in the installation to flicker.

Or it could be some mechanical fault on some motor, causing uneven mechanical load, so pulsing into the current draw. Again those high efficiency permanent magnet motors use to be more sensitive to this, but induction motors do that too.

I haven't noticed flicker being an issue myself with most LED, but I have noticed many places have replaced their fluorescent troffers with LED edge lit troffers and a month later many of them are already eol, yellowing and unevenly lit

I've seen lots of LED highbays and floodlights that have very strong 100hz flickering on the camera.

Another reason I've seen for why such flicker is common on a lot of LEDs is due to the increased use of 'High Power Factor' LED drivers in fixtures and some lamps, which, unlike other LED drivers/power supplies, don't have a large smoothing capacitor on the primary side, but rather have to make up for that on the secondary (to the effect of either adding more or bigger capacitors to the secondary side), which some don't even bother with (as the capacitors can easily end up being the most expensive components in the driver). These HPF drivers have become pretty common, even in residential grade fixtures (even though PF is not a concern in most residential areas), and that's probably in part because they can be cheaper to make due to the fact that they can omit what can be one of the most expensive (and in a lot of cases, most vulnerable to heat) components in the design.

PF may not be that much of a concern on residential installations, the regulations impose power factor/THD limits when the rated power exceeds set value, regardless what the thing is intended for. So going with such single stage ballasts is the simplest way to achieve the requirement.
Plus some systems do not use boost converters, but a modulated current source (so the current follows the mains sinewave) and switches bypassing LEDs in the chain so the LED voltage follows the mains sinewave so the lionear regulator is operated with minimum drop, so minimum losses. This then creates quite complex flashing pattern of individual LED sections (so areas on the fixture surface). But still all is supposed to be a 100/120Hz periodic pattern.

I've a FSL LED filament lamp in my room at my father home, which flickering horribly. It have a linear driver inside without a filtering capacitor or with a very small one.
I think this is the same also with flickering integrated LED fixtures.