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.
IMHO because it means that Signal stays focused on being a secure messaging platform and not an 'application platform'.
Telegram is great because it has bots, etc. but Signal is great because they have one core focus and they don't complicate their infrastructure or codebase by adding things that aren't what people want from Signal.
There's nothing wrong with having two messaging applications; let Telegram be the bot and automation platform of choice and let Signal focus on being a secure and reliable messaging solution.
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.)