It's because most companies that use SMS "2FA" don't use it for your security, they use it for their security—phone numbers are a costly resource, and every phone number an attacker spends to launch a spam attack is a phone number they'll never be able to use again. VoIP numbers flip that logic on its head by making it much cheaper to provision and allocate phone numbers. This also slightly protects user accounts because it makes it harder for attackers to "overwrite" a hijacked user account with their own bogus phone number during takeover, but that's a secondary goal—the main goal is spam reduction