They do that to prevent account farming. Google is one of the very few companies that refuses to accept my VoIP DID number for 2FA, because they know it's not a "real" number. A wise move, considering that SMS-capable DID numbers only cost a few dimes when you buy in bulk.
They're not insisting on 2FA-by-SMS since you can immediately disable it after you prove that you own a valid phone number and thus is much less likely to be a spammer.
They're not insisting on 2FA-by-SMS since you can immediately disable it after you prove that you own a valid phone number and thus is much less likely to be a spammer.