why should the maximum percent yield never be above 100%

Answer 1

because of both controllable and uncontrollable factor such as the during the experiment is conducted certain proportion of the reactant may lost or not fully reacted. It is also because the maximum percent yield is influence by the limiting reagent, the one that decides the amount of product.

Answer 2

I disagree with the above answer since it doesn't address the question. You can NEVER get more than you started with. A yield greater than 100% means you've discovered the secret of making something from nothing. If you grind 15 g apples and 15 g oranges you can get no more than 30 g of product unless impurities were introduced, either accidentally or on purpose.