I tried to set up a Twilio number specifically to handle these services that demand SMS for login.

Weirdly it only works for a minority of services, I expect many use Twilio to send their auth texts and Twilio blocks sending these to their own numbers?

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.

I think some platforms that use SMS 2fa explicitly ban this path for some reason. They can detect it and don't allow those numbers to be used.

It's a really backwards and confusing system, I agree.

No this doesn't work at all. As others have said, many companies will not let you set up an account or send SMS to numbers created this way.

Twitter SMS validation doesn’t even work with my regular phone!

Until you get deplatformed. Tech-first MVNOs don't always have the same consumer rights and anti trust requirements as those regulated by the FCC.