Are there valve caps that show the tire pressure?
These types of valve cap gauges aren’t recommended - either for cars or bikes. They work by bypassing the schrader valve in the tire and using a cheap spring-based pressure gauge in the cap. 

First, they’re wildly inaccurate even on cars. By the time they show red, you’re riding on your rims. 

Second, they’re cheaply made and a significant second point of failure. If the cap is just a bit loose, the o-ring gets just a bit of dirt in it, or the valve cap gets knocked the wrong way, they will start bleeding air. This is most likely why they aren’t sold in the higher pressures that bikes would need. 

Finally, as @Argenti noted, many riders adjust their tire pressure depending on the road or trail conditions or their desired degree of ride comfort. Most bike tires can take a wide range of pressures, unlike cars which have a very narrow band. 

Conclusions: I’d just get used to squeezing your tires (for the lower pressure of mountain bike and comfort bikes) and/or topping off regularly (especially if you have a road bike).