To be fair, Gmail asks for a phone number, but you dont have to add one.

This might depend on the country you're in, but I'm quite certain I've gotten locked out of the signup flow in the past when I refused to provide a phone number.

It depends what you do it from. If you do it from an android device you don't have to. If you do it from the web you do.

I just tried it from my Android phone (GrapheneOS) and it still asks to verify a phone number when trying to create an account via a web browser. (Strangely, even though it's a private browser session it just asks to confirm my number by sending an SMS, not asking me for my phone number like it does on desktop -- I wonder how that works...)

If you're saying that the account creation flow through the system accounts application doesn't require a phone number, how are you sure that Google doesn't just collect the phone number directly from your device (they could even silently verify it through a class-0 silent SMS)?

Does it also not ask for a phone number if you factory reset, remove the SIM card, and do not register the phone with a Google account? Maybe they track the IMEI instead?

I don't think that's why they ask for it, no.

Exactly, just like all those site that added SMS 2FA didn't do it for the extra security.

More than one thing can be true at once.

In the case of Twitter, there is evidence that the initial implementation was meant to just be a security mechanism but later someone else noticed they had a handy database of user phone numbers and decided to treat them as free marketing contact information.