House wired with Cat5e, but no central access?
If you don't have a basement and the builder didn't want to put a cover plate in a closet somewhere, the attic is the only place left inside.

It is very common for a builder to simply run the low voltage wires and leave them un-terminated. A builder can say "wired for cable\internet" without any terminations being made. Hopefully there's an outlet somewhere up there because you'll need power for the router and if you're lucky there's a breaker you can toggle instead of having to climb back up there to reset it.

If you find coiled lengths all piled-up in some location, the builder might have planed on dropping them down one of the walls, otherwise have fun making all the connections hunched over in your attic and dealing with having equipment up there.

Why are most of my cable/coax wires disconnected?

This would be no different than many people's basements I've seen.