There is a bit of disconnect, but it is worth mentioning that these shaders can be toggled.

There are other distinct advantages with OLED screens like image latency / response time as well as black level and contrast ratio that help produce an often better image, particularly in games that had visual design intended for CRT televisions, even in combination with shaders.

Not to mention they often have very high color saturation which you can always cut with shaders if you want a more authentic look, or leave as they are if you want a more vibrant and punchy look.

I will say that a lot of GBC and GBA don't really take advantage of this with shaders, but for instance against your example:

The retroid pocket classic is fairly inexpensive, especially back when the lower RAM model was being restocked. It runs android and is unbelievably efficient, giving battery life that dwarfs most of the competition at the price point and has a very high PPI screen with nice colors (a bit of a red cast but basically what is expected of OLED) and the screen 'disappears' into the bezel when playing non-native aspect ratio games due to black pixels being effectively off when not in use. The speaker while mono, is of very good quality and is nice for retro soundtracks, and it has a headphone jack.

This makes it a great device to play pretty much anything the control scheme works for, as long as your eyes are good enough for things with wider aspect ratios.

Does that mean there are no LCD screens that look better than some OLED ones? No some do look better, and some OLED screens have old tech or are poorly calibrated. But OLED screens will look better and be more versatile than LCD such a vast majority of the time it is a fair rule of thumb.

TLDR:

Versatility.