This all depends on the home. I've lived in homes where the number of bathrooms equalled bedrooms + 1/2. So each bedroom had their own bathroom, and there was a half bath (or two) in more common areas. The nicer the home, the more likely each room will get its own bathroom. It also seems more likely with newer construction.
Often as the number of bedrooms increase there is an assumption that they won't all be used. So having a full bathroom for a guest room is a cost people usually don't want to incur while building. Or the house is built with the idea of parents (who get their own bathroom) and kids, who share and then move out when they're 18. Now that kids are living at home into their 30s, that becomes more awkward.
If you won't be using the extra bathroom, they become kind of annoying. I lived alone in a house with 3 bathrooms for a while. I felt like I had to keep a schedule to remind myself to use the out of the way one, otherwise the water would evaporate in the traps.
