Most MFA exist only as part of an identity product like Auth0 or Stytch.

We only want to use 2FA for specific purposes under specific circumstances. Why doesn't this service exist?

Well Twilio's Verify APIs exist, but they're incredibly expensive ($0.05 / verification + SMS prices). If you plan on verifying users regularly, that's a huge cost.

I cannot be the only one who needs this service (can I?).

So why doesn't it exist? What am I missing?