Many sites ask for your phone just to simplify the "forgot my credentials, help me" customer support calls. Some, especially money-related, want to know your phone as a separate confirmation of your state-controlled identity, in countries where buying a SIM card required legal identification (which likely can be retrieved by a court order if need be).

Yes, SMS can easily be diverted so it's not great for 2FA purposes; a voice call is often an option, and is harder to spoof.

Sites that actually care about your security would go for things like TOTP, or SSH key, or a certificate. These forms do not map easily to your legal identity, but are more reliable proofs of knowing a secret.