Why isn't writing to `PNG:UserComment` supported?

[drgrib]

I can see that it is part of the Python PIL library:
python-pillow/Pillow#4935 (comment)

And both this and PNG:Parameters are written to extensively in diffusion libraries.

But when I try to update this field using exiftool I get

error writing metadata: Error writing metadata: Warning: Sorry, PNG:UserComment doesn't exist or isn't writable

What gives? Why do these fields have widespread usage among the Python community but are treated as impossible to write fields by go-exiftool and command line exiftool generally?