Because it doesn't rely on a single point of failure like centralized services. (You forgot "Ask HN:" in the title.)

Probably because email is so deeply important for the modern internet everyone has a interest in keeping it working smoothly

I always find it funny how many things have tried to replace email and none of them really stick. Slack, Teams, Discord etc are great for real time work but when something actually matters people still send an email.

It’s ugly but it works and everyone already has it.

It works.