That's not always the case. Sometimes it asked me for a phone number, but most of the time not when not using a VPN or something similar. But last year I managed to create two Twitter accounts with the Tor browser and some sketchy email address and never got asked for a number, just had to do some captcha after a few minutes.

I created a few twitter accounts this year for various reasons and all of them had the same number requirement after around 24 hours!

The various Meta properties do this too, except instead of phone numbers they require government ID and headshots. It’s all a scummy dark pattern relying on the sunk cost fallacy.