Inefficiency was always a red herring.

$ curl https://www.reddit.com > /dev/null

    % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                   Dload  Upload   Total   Spent    Left  Speed
  100  970k    0  970k    0     0   840k      0 --:--:--  0:00:01 --:--:--  840k

That's just the index, not images or anything pulled in by scripts. That page takes longer to download on a gigabit internet connection than the <5kB page it ought to be could be uploaded from some asymmetric 10Mbps up cable connection.

The biggest problem is actually funding. The centralized site puts ads on it or tells VCs they could, and then has money to pay for developers or other things that increase the user base.

It's the same reason Linux is used by programmers and not graphic designers. If there is anything deficient about it in the programmer's workflow they fix it for everybody. If there is anything deficient about it in the graphic designer's workflow, they don't know how to fix it so they get a Mac and run Adobe. Social media sites work more like the latter than the former until someone finds a way around that.