This is just a guess, but maybe to simplify the tracking requirements on the ground?
With a minimal satellite system, your tracking needs to follow satellites all the way across the sky, and probably also needs to be able to rapidly make large changes in where it is aimed.
With a very large number of satellites, a ground station might be able to get by with only having to be able to track within a small part of the sky.
If you have enough that there are many of these small areas that always have a satellite in them distributed all across the sky, then even places that have a lot of buildings or trees blocking much of the sky should still have some patch they can view that works.
With a minimal satellite system, your tracking needs to follow satellites all the way across the sky, and probably also needs to be able to rapidly make large changes in where it is aimed.
With a very large number of satellites, a ground station might be able to get by with only having to be able to track within a small part of the sky.
If you have enough that there are many of these small areas that always have a satellite in them distributed all across the sky, then even places that have a lot of buildings or trees blocking much of the sky should still have some patch they can view that works.