I don't understand, all they have to do is tap submarine cables, why is that infeasible now? What specific thing do you think they were collecting before that they can't now?

Metadata is extremely valuable!! lots of things can be inferred from it. In other comments I've decried companies like slack including your password reset or login codes in the email subject for example. They can take any packet and trace it back to a specific individual, even if you're on Tor, chaining VPNs,etc.. without decrypting it. They can see what destinations you're visiting. they can build a pattern of life profile you and mine that. The ad industry does much of this without access to global internet traffic captures already lol.