Why don't U.S. SOX systems  require an ignitor ? 

The need to use 120V supply (at least as an option) makes the ballasts already more complex, a bit more lossy so designing them so they can directly ignite the lamp does not pose any additional burden.

Because the low pressure concept was older and the lamps so efficient, it made sense to design the ballasts as more complex with higher OCV (various topologies from resonant boost, step up autotransformers,...) even in 230V areas, even when it meant higher ballast losses at that time. Only later the semiconductor technologies allowed to design "ignitors" capable to boost the OCV enough to work with a plain series choke as the main ballast. Because the ignitor is normally not active during operation, the lower losses of the simpler ballast made huge saving, so the industry went that way on newer systems. But still it relied to the 230V mains, it won't work with just 120V without a step up transformer. And once a step up transformer function gets involveed, the losses become not that different from a ballast designed to work without an ignitor, so the complexities related to the ignitor won't pay off that much.

And there is another aspect: In the US, generally all discharge ballasts use to be marketed as rather encapsulated units (either physically, or at least marketing wise, distributed as complete kits). So even when the ballast box actually does contain the ignitor, it is very likely it won't be mentioned.
On the contrary the European marketing style uses to offer individual ballast components separately (separate ballast choke, ignitor, capacitor), very often with lantern assembly or servicing each component even coming from different manufacturer. So in the 230V world the need/existence of an ignitor just can not be hidden the same way as on the "120V" market.

Probably thanks to you guys using CWAs😎, auto transformers can pretty much start any low pressure lamp, and as Medved says, they are slightly more lossy, and there was a drive here to reduce those losses by using simple chokes.

Historically SOX lamps were run on leak transformer. These are transformers with very high open circuit voltage (up to 800V for SOX 180W). Due to air gabs the impedance of them is however very high. The high OCV allowed a lamps ignition, similar to HX ballast with probe start MH is the US. due to the high impedance of the transformer the voltage across the lamp would drop to the normal known lamp voltge. this also the reason a wide variety of SOX lamps can be used on the same transformer. When these lamps were first marketed in the 30s, there were no electronic ignitors, so apart from Osram/Philips self starting design this was the only way to ignite th lamp.
However these transformers are rather lossy. SOX lamps below 90W require also a lamp voltage of about half of 220V or less, allowing operation with cheaper and more efficient chokes. However then an external ignitor is needed to light the lamp.

 Here's a photo of the label of one of my SOX ballasts : https://www.lighting-gallery.net/gallery/displayimage.php?album=5203&pos=88&pid=163798. So the high (610-760v) OCV negates the need for an ignitor ?

There were no electronic ignitors but bimetallic starters sure existed, so might be adapted.

Leak transformers are sure very straightforward to use and robust, but you pay a high price of great bulk, extra cost and most significantly, rather poor efficiency negating one of the most valued advantage of SOX, high lm/W.

That is especially evident with small American wallpacks using 18W SOX like some pictured on L-G. While bare 18W SOX lamp is still not bad on lm/W, system efficiency is just meh, dropping to the level of a just good CFL....

So buyers certainy asked - why bother with that rare expensive lamp having a strange monochrome light while I can stick a simple CFL whigh gives almost the same light in white and runs directly off the line, and even more, replacements are always at the supermarket nearby

 

In Switzerland all SO (SOX) installations were leak transformer circuits. In the time where SOX lamps gained popularity there were no circuits with ignitors available yet. That’s my guess.