Because Google knows best.
I hadn't considered MFA because I use Authenticator and have most of my TOTP seeds in 1Password anyway, but man I'm thinking I dodged a bullet because there are a handful of things that use SMS only.
I just bought a prepaid card only for this - Google, Facebook, Twitter, Instagram. I don't use it for anything else. I took it out and hope to remember to top it up for another year to keep it in service. It's so stupid.
Nothing mysterious. Authy and Google Authenticator simply implement the same standard, RFC 6238.
SMS 2FA is a different proposition, in that the authentication provider sends you the intended response directly. This is a little easier for nontechnical users in that it doesn't require an app that can compute TOTP responses, but it's markedly less secure in that anyone receiving or intercepting the SMS can immediately respond to the challenge, and less resilient in that you can't authenticate at all if you don't receive the SMS - which is the failure mode under discussion here.
Maybe they've fixed that since then, but I don't care. I'd rather risk a theoretical flaw in Authy's E2EE than risk getting screwed again, and by now much more thoroughly, by the flaw I know about in Google Authenticator.
Today I have to enforce MFA at the small company I'm working at, and just a month prior I held a cybersecurity seminar for them and explained that "phone MFA is… okay… but you should really use verification apps." But I didn't realize that you can't even enable authenticator-app-MFA with Google without doing it through a phone first… just infuriating.