Charge Pump for P-Channel MOSFET
It would certainly be possible to use a charge pump to generate a negative gate voltage.

However, N-channel MOSFETs are cheaper/smaller than their P-channel equivalents. So when you have a charge pump anyway, you could modify it to generate a positive voltage instead, use an N-channel MOSFET, and save some cost/space.

In other words: a high-side P-channel MOSFET makes sense only if you can drive its gate directly and thus can use a simpler circuit.