Alt-text on media: Why not compulsory on some instances?

I don't have many fedi accounts, but looking at public Mastodon feeds it is very common to see people requesting others to add alt-text to their media and getting a lot of boosts/etc.

Is there any reason (beyond a very mild convenience) for some Mastodon instances not to require alt-text on media? It seems like something a lot of admins would want to do, given their general audience, and naively I'd say it's very easy to implement.