Why don't showers have shut-off valves?

Mine does. They are usually easy enough to add.

Yeah, depends on what state you are in, but most states require shut offs to be installed on shower valves. If they are there, they are usually inside the ACCESS PANEL on the reverse side of the shower valve (usually in a closet) on upper floors or just below the tub or shower in the basement on 1st floor bathrooms.

Many new tub and tub/shower valves now have what are called "integral shutoffs"...means the shut off valves are built right into the shower valve BODY (usually just under the handle and trim plate assembly) and are shut off using a large flathead screwdriver.

Mark

I'm a valve freak. Some places I have valves which shut off valves. Can't have too many valves or unions. :D