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.