Why? Bots using the bot API can't send unsolicited messages to people. (Bots that control regular user accounts without using the API can of course do this, but Signal's not immune to that either.)

Not having a bot API is what stops me from moving to Signal. I have 10-ish telegram bots running for myself, I just use telegram as UI for a script running on backend, and results are easily shareable with my other telegram contacts. I have no clue why not having this is amazing in any way.

Bots are separate accounts, marked as bots and cannot message anyone first.