It's because they think AI will make them rich somehow and Signal is just a "boring" chat app.

But to be fair Mail is pretty centralized too nowadays for most people @gmail and @outlook. Of course there are others but my point is that there a a hand full of providers up to a point that it's almost unfeasible to maintain and run you own mailserver because there is a high chance that those large providers will flag your mail as spam because it's rare to see a "wild" mailserver.

Nothing like a mall, and your examples don't affect usability.