Physics and requirements.
A mobile phone has the requirement to be, well, mobile. As such there’s not a lot of space for large antennae. If you look back at the earlier mobiles they had (extendable) external antennae (eg. the [Nokia 3110](https://en.wikipedia.org/wiki/Nokia_3110)). But if you want to be mobile you also can’t use a directional antenna since these have to be pretty finely aligned. Now combine the small form factor with an omnidirectional antenna and you get very low transmission and reception power.
On the mobile towers you don’t have that issue. On those you can install big antennae that can capture very tiny signals from all the noise around it (and modern setups can even do tunable directionality through [MIMO](https://en.wikipedia.org/wiki/MIMO) configuration, improving on that even more) _and_ you can push a lot more power into those for transmission.
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