I never get any spam on my chats

I've never crashed my car, should everyone get rid of their car's seat belts?

Your experience does not represent the world. I've only experienced 2 cases of spam on Signal, but they were all within the last year. I've had zero spam in the many years I've now been using Signal. So, while my anecdote is just as invalid as your single point of data, there's definitely a trend for increased spam as a service gains popularity and it makes sense that they're looking at enhanced methods to block spammers.

I still don’t see why they want a super secure smart system to block with captcha

You don't understand why Signal, one of the most secure messaging platforms available, wants a super secure smart system to block spammers? I think you answered your own question.

Telegram for example you can add your own bot to kick the bot users. If you get a direct message you can just block and report

Telegram stores all your data and can view everything you do - unless you opt into their inferior E2EE chat solution known as "Secret Chats" - so it's easier for them to moderate their services. When you report someone, Telegram moderators see your messages for review [0] and can limit an account's capabilities. Signal can't view your messages because everything is E2EE, nobody but the intended recipient can view your messages, they can't review anything.

As you can see, without even digging into it too much, I've already found one case where Signal faces challenges not present in Telegram. Thing's aren't always as simple as they seem. Especially not for Signal, as they've worked their asses off to ensure they have as little data on their users as possible.

[0] https://www.telegram.org/faq_spam#q-what-happened-to-my-account