The reason for this is that in PCB design, layers are always processed in pairs. So if you want signal layers, you immediately have to take at least 2 (top layer and bottom layer). Single layer boards aren't manufactured anymore in standard pools. Same goes for power planes: they're also added as a double sided copper laminate, so you also immediately add two.
Of course, nothing prevents you from keeping a layer completely empty. This will then have the same result as the layer being absent, although the PCB might be slightly thinner. However, this is usually not a problem except for controlled impedance applications.