[dimmke]

The reason most services require a phone number is so you can't just create a new account if you get banned and ideally your account is somewhat tied to a real person. They ban VOIP numbers because it would defeat the whole point.

[TwoBit]

What's preventing someone from getting mutiple regular phones and accounts for them then?

[closeparen]

Cost. You can do it, but not hundreds of times like with an email address. Probably not even 10 times before you give up and do something else.