They’re covering the planet for cell coverage with 20 satellites? Why are they launching 1000s? Why do we have so many cell towers?
But also: Capacity. The more satellites you have, the smaller you can make the cells they're projecting to earth, and the higher transmit power levels can be (assuming that the satellites are largely power limited), as well as the better the SNR is going to be as you can focus the same antenna payloads on a smaller area.
A beefier satellite with larger solar panels, more antennas, and bigger and reflectors could achieve the same with fewer satellite vehicles, which seems to be the approach ASTS is taking.
But Starlink, as far as I understand, is also providing their "regular" beamformed service using the same satellites, and there you can actually increase capacity greatly with more satellites than strictly required for global coverage (since the terminals can also "point", i.e. you and your neighbor can use the same frequency at the same time, but pointed at another satellite, increasing total data rate per Hz).
Line-of-sight is a key reason. Satellites have fewer obstructions compared to terrestrial towers.
“Bandwidth per beam is only ~10 Mb, but future constellations will be much more capable.”
They are not. Of these 20, 13 were the last to launch of the first shell of the direct-to-cell constellation, which contains 285 satellites. Once they reach destination orbit (which will still take weeks), Starlink DtC will be able to provide continuous low-throughput comms.
> Why do we have so many cell towers?
There is a superlinear relationship between total system capacity and the amount of cell towers. The total amount of capacity that Starlink DtC has will be, frankly, pathetic. 10Mb/s sounds good until you realize that that's all the capacity of the beam, that's spread out over a huge geographical area.
It will not be replacing cell towers. Instead, it will just end coverage gaps for low-throughput workloads, like sending emergency messages.