Its a technical problem. The light output of LEDs at low voltages is much more varied than at higher voltages. A bunch of identical LEDs at a high voltage may have a similar brightness, but the lower the voltage goes the more different they will be, till you get the point where some be brighter, others will be dimmer, and some may not light up at all. I believe this applies to OLEDs as well, and most likely MicroLEDs if they become commercialized.
Accommodating this without PWM will lead to higher costs / low production yields. Presumably it is possible though.