It's the cell network. The basic problem is that adjacent cells need to be in tight sync for time division agreement and handoff, but the obvious algorithm of "

It's the cell network. The basic problem is that adjacent cells need to be in tight sync for time division agreement and handoff, but the obvious algorithm of "just sync to your neighbors" is highly problematic at scale because even if a big group of towers does a good job of drifting together there is always a boundary and the problems will just happen there. Since the cell network is highly profitable infrastructure, they just used the overkill strategy: "GPS sync everything and put atomic clocks everywhere for good measure." At least hobbyists get cheap and plentiful Rubidium clocks out of the deal! I don't know if newer standards are still as sensitive about timing agreement but it was definitely the case for a good long while.