Why aren’t there water shutoff valves for each room?
This is a result of building as efficiently (cheaply) as possible, and there is almost no need for room valves.

First, efficiency:


Valves are more expensive than pipe. Extra valves means more labor cost.
If the valves are to be centrally located, then more pipe is required because each "zone" will have to be home-run instead of branching off a shared pipe. Extra pipe means more labor cost.
If the valves are not centrally located, then they need to be located somewhere accessible to be utilized. That means either designing a space for them, or locating them somewhere not in the way. Extra design work and extra pipe means more labor cost.


Second, there is little need to install valves in advance:


In an emergency, shutting off the water to the house for a couple of hours (or a day) while waiting for a plumber (or a run to the hardware store) is an inconvenience that most people can manage.
The most common leaks are at fixtures, not in pipes. Each fixture usually has its own shutoff.
If you need a valve to facilitate a renovation while the house is occupied, it's easy to plan to shut off the water to the house briefly to install a local valve.


In summary, the expense of the extra valves, pipe and labor are not worth it, especially from the perspective of a builder, where a small reduction in cost per home built results in a large savings overall.