That location information is not available to apps or ad networks without user consent. The government can access it from the carrier with a warrant, but that's not what we're discussing here.
Carriers have also sold customer location data, no search warrant required. Though we can rest assured that the FCC has slapped the carriers' wrists with the utmost seriousness.
IP doesn't handle roaming very well. If you got routed onto the internet directly from your local cell tower, then your connections would drop whenever you switched to a different tower, which is somewhat suboptimal. Cell networks handle it at a lower level and route your traffic through a central location which serves as the origin of your IP traffic. Geolocate your IP while on cell data and you'll probably see something pretty far away from where you are. My phone's IP address at the moment is about 400 miles away from the actual phone.
