The phreaking [1] community was huge and becoming increasingly sophisticated long before mobile was even a thing. I think it's mostly that telecoms were traditionally discouraged from pursuing security. There's, at most, a minimal commercial incentive to it, and the government loves comms that can be easily spied on meaning you're going to get pushback from that side if you start aiming for security.
The idea to start using SMS for secure purposes was similarly probably never really about security, but an advertising/government driven effort given that it helps create a fairly reliable tracking identity for a person. It makes no sense otherwise to use SMS over something like a 2FA app which is completely cross platform, secure, free, and has basically 0 downsides relative to SMS, and a whole bunch of upsides. The only thing is that it's also anonymous.
The idea to start using SMS for secure purposes was similarly probably never really about security, but an advertising/government driven effort given that it helps create a fairly reliable tracking identity for a person. It makes no sense otherwise to use SMS over something like a 2FA app which is completely cross platform, secure, free, and has basically 0 downsides relative to SMS, and a whole bunch of upsides. The only thing is that it's also anonymous.
[1] - https://en.wikipedia.org/wiki/Phreaking