Because that's not a solution: the post remains inaccessible. Unless you edit the post, demanding they fix it is the only solution.
I see stating the obvious struck a nerve for the downvoters, so let me dumb it down for the unfortunate ilk unable to grasp the obvious: y'all full of shit. You think someone scrolling through posts the same as you is going to notice a text alternative hidden away in some comment on some other page? You expect them to open every post unlike you for something that likely won't even be there? That's not equivalent in the slightest. Shit's daft as fuck: keep dreaming.
Now examine critically the substance of the other commenter's actions. While the comment is effective at drawing upvotes, is it actually effective at the main outcome of doing anything meaningful for accessibility as they purport? No, the comment doesn't appear to anyone scrolling through posts. Tellingly, the community is simply rewarding self-indulgent ploys to gain upvotes that do nothing for accessibility while condemning meaningful answers to actually solve it. That's quite damning.
It indicates a toxic community that cares more about indulging community narcissism than acting on their purpose directly within their capacity right here to address an issue in their control. A real progressive would care enough about equal access to look up & determine the truth on the matter, so they can pursue and promote real answers instead of empty gestures that don't meaningfully address inaccessible content. The placebo they pretend works stops them from developing real cures.
In sum, some lemmy communities in particular are toxic & hostile to web accessibility in ways we don't find in bluesky and other parts of the fediverse such as mastodon where web accessibility is common practice.
Back to the point, here's an introduction to web accessibility that you seem to sorely need. You can also read the technical guideline while you're at it. However, your nonsolution is an unhelpful illusion that diverts people from pursuing credible solutions.