The point isn't to authenticate control of an account, it's to tie the account to some kind of expensive-to-replicate real-world cost, ideally one that most potential customers are already paying for.

Phone numbers are nice because the marginal cost to a customer is low (they probably already have one) while the marginal cost to a bad actor is high (it's expensive to acquire many of them or to change one once it's been identified as malicious).

My current phone contract: 3.99€/month

My current twitch subscriptions: 11.97€/month

I can’t really see how they need my phone number to make it too expensive to be a bot.

And if that is the thing, then that’d makes them even more shady, claiming it’s for account security when it’s for their bit protection.