There’s some tradition and exceptions to the rule which I can elaborate on later.
But the simplest answer is because in a car, the steering mechanism is at the front, while in a boat the steering mechanism is at the back. Mechanically, keeping the driver close to the steering mechanism has benefits (smaller mechanism, lower weight, less of a problem to extend steering to wherever they are sitting). And for the driver, being as close as possible to the point of turn makes spatially understanding driving s lot easier.
In a car, the front wheels turn to turn the whole car.
In a boat, the rudder (which is what does the turning) sits in the back of the boat at the bottom. You could design a way to transfer the power of the steering wheel from the front of the boat to the back, but this is a larger engineering challenge and could mean cutting back on usable space inside the boat. The easier solution is to put it in the back.
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