The answer is capitalism. Doing things the best way and most profitably is not always the same.

Laziness more or less.

It's easier and cheaper to just run one wire. Most people don't want more than a couple different port locations so why bother?

And you're not installing the other two items so it's not your responsibility. You're also not going to get paid more for doing the extra work. Even if the homeowner was the one paying you, most aren't willing to pay a bit extra just so it's easier on the next guy.

This is interesting. I did my garage from the studs out. Using conduit to run electrical never even occurred to me. I think it would be a real bitch to drill holes through each stud level enough to push a rigid conduit through. Maybe there are some flexible conduits, but now you're taking a 1/2" hole and making it a full inch, in a 2*4, which is nominally 1 and 3/4". But now I'm thinking they did it with Bx, and maybe I'm not fully versed on why. I know the shielding acted as a ground as well.

I'm rambling. I think there's some rationale that's not just capitalism=bad. Code is okay with running straight Romex through studs, so long as you staple and whatnot. Maybe it'd be above code to use conduit, but I think it's just a lot of extra work for not very much payoff. So yeah, capitalism=bad, I guess.