I think conversion rate and support cost associated with 2FA-OTP are worse enough for SMS to still be worth it, especially as a phone number also gives you a good marketing ability and a reasonably unique identifier for a user.
That is what everyone dances around in these discussions. It doesn't matter if it is a good second factor because it is an excellent user tracking identifier and that is what they were really after. Twitter and facebook both lied about only using these numbers for security and then almost immediately put them to use for advertising purposes. We only know about it because they were big enough to sue, I'm sure every crappy site that gets the number sells it. As a bonus, it also allows them to dump a lot of the infrastructure and support problems onto some one other than themselves.
The biggest problem with SMS-2FA in my opinion is a lot of places are setup so it isn't even a second factor. I can often reset my password just through email so it just seems like throwing a threadbare blanket marked security over the top of a user tracking scam.
If not, everybody would be using OTP already.