because of the beefy current absorbed by the starter motor.
DD, think of it this way; you can't operate the starter until the master switch is on. So the master switch also controls the starter (though don't ever do it, you could hold the starter engaged with the master off, turn the master on, and get cranking there and then. That's why it is prudent to consider the prop swing area when you turn on the master, just in case of the very remote possiblity of a stuck starter soliniod).
The starter motor is connected to the battery (or ground recepticle) by wire to size of a finger. That wire does not go up to the back of the master switch, so you know there's a solinoid (relay/contactor) in that circuit somewhere. It's usually very close to the battery to keep the runs of big expensive heavy wire short.
As stated, generally, the operation of the master switch is to ground the soliniod coil, rather than to energize it. It is for this reason that the soliniods used in these circuits are special, and not interchangeable with those typically found in automotive applications.
But, I have drifted this from a fuel leak thread. Fuel in the cabin is a necessary design element of every powered airplane I can think of. Leaking of fuel at all, much less into the cabin, should not be accepted.