Is there any particular reason why I never see the MOSFET between the cathode of the LED and Vin?
I think you mean "between Vin and the ANODE of the LED". Cathode is the negative side: ground.
Yes. It is easy to use a logic-level N-Channel MOSFET as a low-side switch. Put it between the load and Ground and connect the data pin to Gate through a current limiting resistor.
To use a MOSFET as a high-side switch you need a P-Channel MOSFET and you need to raise the gate to Vin to get it to turn off. You can do that with a pull-up on the gate and an NPN transistor to drag it to Ground when you want to turn it on but that's two more components.