Using transistor as switch, why is load always on the collector
A simple reason to have the load on the collector is that it keeps the base current independent of the load.  That makes it much easier to reliably keep the transistor saturated.

If the load is on the emitter, then the base current depends on the load.  If the load is an LED, then the voltage you have to apply to the transistor base to reach the needed current goes up by the forward voltage of the LED.

If the load is a motor and it is connected to the emitter, then the base current depends on the motor, and will vary all over the place as the motor turns.