I wonder who thought it was a good idea in the first place. Maybe their real intent was to collect people's phone numbers instead of protecting their accounts.
Another benefit is that they can block duplicate accounts with the same phone number, this effectively adds a cost to account creation which can help to reduce spam.
And yes, some will then use it to spam or sell.
I don't think anyone ever thought that SMS 2FA was strong protection. It mostly benefits the service operators by reducing spam and stolen accounts.
Kinda like SSNs though, I think it's way past the point when a better designed system should have been developed. Sadly, that costs a lot of money.