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.