It's largely because of how Kailh had designed their sockets. They need larger holes in the PCB than for simple switch pins, and the soldering pads are surface-mount to the left and right side of those holes, not north and south of them.
That makes it more difficult to design a PCB with horizontally overlapping footprints for sockets.
Now there are a few keyboard PCBs that support multiple layouts with sockets, but they tend to have one variation's socket north of the centre, and the other socket rotated 180°, south of the centre.
I've also seen experiments with sockets rotated 90°...
Desoldering comes with a larger risk of messing up, and surface-mount pads are more difficult to desolder than through-hole switch pins. And the whole point of sockets is to avoid soldering yourself anyway...
An alternative to Kailh sockets are Mill-Max or Holtite sockets - each of which is only small enough to fit inside a pin-hole in a PCB. But the PCB needs to be have been designed with closer tolerances so that the holes are just the right size for the socket to fit. The sockets are also not as durable.
(Not that Kailh-style sockets don't have their own durability issues... Some enterprising switch company could better design a better socket instead of just copying Kailh like how Gateron and Glorious did)