It's easy to forget this fact (or completely ignore it) but cellular phone services are nothing but rather complex, but very low power two-way radios. As such, service quality and signal levels are as affected by interference, geography, vehicles, people, other objects, building materials and structural density, interference sources and even weather in the same way any other radio technology is. The only real difference is that while your local radio stations in the FM2 band transmit at megawatts of power so have a range of maybe 100 miles, cellphones and towers transmit at milliwatts, so have a very limited range and are, unfortunately,
more prone to degrading influences due to the low power levels.
Added to that, the frequencies at which cellphones work are so high that they are easily deflected such that in towns they can bounce between and around buildings causing multipath distortion which serious degrades signal quality, readily absorbed by structures, and prone to the disruptive harmonic transmissions of other services such as paging systems - which are typically poorly policed by the FCC. Even when cellular carriers
want to plug service gaps where they know it's needed, they often find themselves being delayed by local communities who don't want a cell tower in their neighborhood, and by complex regulatory requirements, even if they can find a suitable site.
Cellular service is far from perfect as a result. And sadly all the carriers are much the same in that in some areas they work well and in others not.