The answer is because..... .And there is little technical justification for some of the rest!
Pipers - even the high wing ones usually have separate tank feeds - the technical justification is obvious when you're dealing with a low wing and surprisingly similar for the high wings - the right hand tank (usually) has more convoluted tubing runs and hence poorer fuel delivery and water seperation - hence the need to use the left tank in low fuel conditions.. .Cessna simply plumb to two tanks together - easy on/off cock and the fuel delivery is not a problem. Of course decanting from one tank to the other can be if the aircraft is parked on sloping ground.. .My Auster has both wing tanks plumbed together and then has the addition of non-return valves! (well built on a cost plus basis for the military so what do you expect!).
Interestingly even with one tank nearly empty and the other half full these valves do not offer enough resistance to starve the engine (its one of those things you try when you become aware of it - don't worry there was fuel in the belly tank!).
So as I started not a lot of logic and almost as much common practice or company policy. In most cases the fuel system is however too complicated and has rubbish gauges - I wonder if this is why people still run out of fuel?