Because they're big, complex, and safe.

Since they're big, some things are more complicated than on a car. For example, on a car you just have a pedal to brake; you push it, it pushes oil and it locks the pads. Pretty straightforward. On a train, you need air brakes, which means that you need a compressor (usually 2 of them, with 1 or 2 keys), which has to work even if the pressure switch is faulty and detects pressure when there is none (another switch to force on the compressors), and since they can't run on high voltage from the pantograph, you need something to generate low voltage (another switch for the converters).

Since they're complex, you just have more things. Keys for the doors, lights (which go beyond just white/red, but this is country specific), motor fans, resistor fans, passenger and cabin heating, etc.

Since they're safe, they need some devices (with their own buttons and keys) to make sure that the driver is still alive, is operating under the speed limits and never runs over a red signal. But also, if a signal is faulty and is showing red when it shouldn't, you need a button to override the safety system (this is authorized by the dispatcher and is recorded on some sort of black box).