Why are Vegas pools so cold?

They definitely do. There is no way you could keep a shallow Vegas pool from warming up to bathroom temps on a 100+ day without some kind of active temperature management. My lake heats up to bathwater warm in July and it's 100ft deep and we only get in the 90s tops here in Michigan.

 

With low humidity and hot temps the water will evaporate quickly and need to be filled at a faster rate than say in Chicago. The number of people will also cause the pool to change out water faster depending on what the PH levels are. You can only add so much chlorine or other chemicals.