Because disposable emails are trivially used by casual users and companies see phone number as harder to falsify (correctly, IMO). It's not impossible, obviously. We know SMS can be hijacked and spoofed, but the casual user won't be doing this, today.
If the point of requiring an ID is to prevent abuse, why do I care about what casual users are doing? What I care about is preventing signups from people who want to abuse the service, but presumably making things harder on casual users doesn't work toward that goal.
Still, it raises the bar.
But why phone? Why not something more specifically set up for the purpose, like Google authenticator or 2fa keys?
So many companies are under the impression that a phone number is a unique, secure identifier for their users when in fact it's fairly easy for a knowledgeable attacker to hijack a phone number for calls and SMS.
Things are always fairly easy to the knowledgeable or experienced. It’s definitely a better practice then not asking. Lots of medium sized businesses can’t implement a cyborg biometric MFA solution. So they easily ask for a phone number. Most don’t make it mandatory, but I think generally it is the best current realistic solution.