Because good alt text needs a lot of context, so it must be done by humans for humans at our current state of tech,

Because good alt text needs to be highly context dependant, so you can’t automate it. The better alternatives we have right now are crowd-sourced alt text sites, where volunteers may generate descriptions.

Theyre adding an opt-in alt text generation for blind people

No, that’s not useful at all, but Mozilla refused to listen to the blind community.