The distance at which 5G works reliably is a big part of this, but saturation is also a large contributor to the problem.
With the proliferation of smart watches, cell towers are getting overloaded as more and more people are now walking around with two devices trying to keep connection with towers.
A lot of people have non-cellular smart watches that only connect to their phone via Bluetooth, but an increasing number of people who cannot or do not want to afford an up-front purchase of a $200+ device will turn to their carrier to buy a smart watch just so they can pay over a couple years. Most people probably rarely, if ever, use the cellular connectivity of the smart watch, but the carrier isn’t going to subsidize the cost of a watch without an active plan, so the number of cellular-connected devices per person is growing, even though the smart watch connections go largely unused.