Even a couple of dollars more for a contractor is a lot when a thermostatic valve doesn’t add any value.

You can get inexpensive tankless water heaters.

You want to open the hot valve 100% until the hot water comes because it conserves water. You want to clear out the hot water line as fast as possible. Turning it to 70% just takes longer to heat up and you’re just dumping cold water out for no reason.

Might be popular in new expensive builds

Standard house isn’t going to have them

Really I don’t see much of a point for one. Crappy house with crappy plumbing, but they aren’t going to buy a fancy valve to make the problems somewhat manageable.

Tankless water heaters are becoming more popular. The thermostatic valve would save you from turning the standard water mixer a little hotter when you run out of room temp cold water in the lines and the cold water from outside reaches the shower. Otherwise, you are always being supplied constant temperature water.

I feel like most people that would be willing to pay extra for a thermostatic valve would pay extra to have an oversized water heater, or even tankless.