Why are fuel pump switches separate?
Since engines are run only when the fuel pumps are on and vice versa&lt;&lt;
  


Not true. Engines can be run without fuel pumps, to allow for fuel pump failure. 

The need for fuel pumps depends on the location of the fuel tanks relative to the engines, and the relative thirstiness of the engines - there are some airplanes where the fuel pumps are only used at some times (i.e. near the ground).

So it all depends.