I figured it out.

My specific issue was because I'm using the Retroflag GPi case. The case's screen requires a patch to be installed to use Retropie. I forgot about it because the Recalbox has an install package specifically for the GPi case (which is super nice) and it just works with the screen/buttons. I've re-flashed Recalbox a few times and I forgot to manually reinstall the patch when switching back to RetroPie.

@Scavy They both flash the the micro SD card the same way. Both are through the raspberry pi imager and both have a pop up claiming to format the card on flashing.

I've had it work one way and not the other, e.g. running RetroPie just fine, formatting and flashing Recalbox, running recalbox just fine, formatting and flashing RetroPie again, raspberry pi no longer boots, formatting and flashing Recalbox, recalbox works fine.

I asked on this forum because it seems like a simple issue with Recalbox I just don't understand.

I'm using an A10 Micro SD card and I installed Recalbox on it. It works fine but I tried flashing it again with RetroPie and it won't boot. When I flash Recalbox back onto the card, it boots just fine.

Do I have to format the card manually in a specific way?

I tried installing Recalbox on my GPI case's Raspberry Pi 0 W. I can't run any of the preloaded games and there is no share folder. When I reboot the system I get the "installing GPI case" screen and creating share folder screen.

I heard the size of the micro sd card may be an issue. I'm using a 256GB card. Here's a link to the specific one I'm using:

https://www.amazon.com/dp/B07S5T7YR5?ref=ppx_yo2ov_dt_b_product_details&th=1

Is it just too large? Why exactly would it matter?