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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.

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

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.

Part of SMS verification is to raise the cost of Sybil attacks. Well known sources of cheap bulk phone numbers are often banned for that reason.

I'm not sure if Twilio blocks sending to their own numbers but you can't receive from or send to short codes, which will limit you a lot when verifying with services like Uber.