Because they require the same amount of space that a model would. If the build plate can only fit four models, adding prime towers reduces that to two.
The reason for the space around the model when in by layer model is so the head doesn’t accidentally collide, this would be true of the prime tower as its height is not always equal to the currently printed object and always higher than the subsequent models that are printed.
As I said before, it isn’t impossible, just a lot of issues in providing it and they work (as most businesses p) by proving the most features to the the most users who will benefit.
The other factor at play depends on which printer is being targeted. The a-series can only print very short models in by-object mode vs a greater height with the P & X series printers.