Question

why should the maximum percent yield never be above 100%

Answer 1

have you ever seen output more than input i guess NO therefore yield can't be above 100 percent...it may reach to the 99.9999% but can't be 100% because conditions can't be ideal

Answer 2

Remember the Law of Conservation of Mass? The law states that in any physical or chemical change, mass is conserved. So the total mass of your reactants must equal the total mass of your products. The actual yield can never be more than the theoretical yield. The law of conservation of mass would be violated.

Answer 3

you cant get something more than the input. here's the analogy :

lets say u take 2 apples and add it into a jar with another two apples, will it be 5? No.

there are no output that will achieve 100%, due to the defects and surroundings condition. like ghazi said, no IDEAL condition that could be achieved.

:) hope this brings something to mind. xD im not good at theory but maybe explaining in other way, i could.