GPS's precision costs a lot of battery. There's an API to ask for location, and the more precise you ask to be updated, the more battery it uses. So most apps opt for a less-precise location and stitch it to road maps, assuming you're at a particular location.

Also, there are many spots where GPS doesn't work as well--it is, after all, relying on Signals From Spaaaaaace that can get blocked by buildings, other signals, weather, or nearby military installations...