I'd expect a not insignificant amount of image hosting sites to throw away unrecognized chunks, especially if they contain a few dozen kilobytes of data. It might even make sense for privacy or security reasons, just like EXIF data is usually stripped.

One core concept of PICO-8 is the artificial limit to 32 KiB per cartridge.

While that would certainly also be enforceable with PNG data chunks, I think the 32K restriction lends itself to the steganographic solution more naturally than using a chunk would.

"why bother" isn't a very relevant question about anything PICO-8. The entire thing is about artificial limits for "because we can" reasons.

Basically, the answer would be something akin to "because that would feel like cheating"

As other posters mentioned, using extra chunks might make it incompatible with hosting sites that remove them.

Plus consider that they can be removed even perhaps unintentionally, if the upload process manipulates the image, it might convert the PNG file into a format-independent "Image" structure while in memory, which possibly discards this extra information.

Steganography is a bit more resillient since it will survive basically everything but scaling or lossy compression, as the information is in the image itself (and thus a format-independent program doesn't need to take into account that data).

Naturally, it is still liable to websites that assume images are photos and thus can be scaled and otherwise tampered with much less consequence.

“Because Zep thought it was more fun that way” is always a good guess at the reason anything in Pico-8 works the way it does, tbh.