Even if they use a shitty cable you can pull your own cable through after the fact, no? Might not work well with flimsy fiber if they took tight turns, but CAT7 should be fine.
Often you can't use the old (unwanted) cable to act as a "fish tape" or "mouse" to pull a new cable because the old cable is routed through many small holes in 2x4 studs, ceiling plates, or joists. The path usually has several tight 90 degree turns, with staples, and some of the holes are even spray foamed. There would be too much friction inside the walls to pull a cable for non-trivial distances.
If the old cable was inside of conduit, pulling a new cable could work but that type of install is very rare. Conduit for networking cable is typically only used underground to bury the cable between buildings rather than inside the walls of a single home.
In my previous house an upstairs leak resulted in a downstairs ceiling coming down which was a boon for wiring up most of the house and adding in ceiling speakers and can lights.
And if you are retrofitting a cable run, yeah it probably will be 'just hanging freely behind the walls'.
From builder's perspective it is "controversial" to run conduit because they're trying to tick boxes as cheaply/fast as possible and conduit is the exact opposite of that goal. If you want conduit, I'd assume you have to pay for it as a bespoke upgrade.