Question regarding Orientation of P-Mosfet in Arduino/Microcrontrollers

I was wondering why in the Arduino schematic, and other microcontroller schematics, the P-Mosfet is connected such that the Drain is to the USB while the Source is to the input of the LDO, and not the other way around?

From my basic understanding of how a P-Mosfet works is that current flows from the source to drain.

You can see the full schematic here

Thanks for the help in advance!