What would be the point of a book where the larger army wins. So there are these two sides, and one has the bigger army, and at the end of the book, they do the logical thing and wipe out the smaller army. And then the book is over. Sometimes there are things that happen in most books, and in most instances worth reporting in history. Austerlitz would not be nearly as important a historical battle if Napoleon had had a force a billion times the size of the other guy and crushed them without a second thought. Austerlitz is important because he took his smaller force and won a decisive victory with it. Most books are written about times in the world that are worth reporting, and the larger army doing what it usually does is not worth reporting.

Also, just becuase it is often in books does not mean it is overused. Is the happy ending overused? Maybe, but it isn't going to stop people from using it, or stop people from reading books with happy endings. Most fantasy books are written about important times in another world, just like histories only with dragons. Would you really like to read a book where the bad guys have this massive empire and the good guys are nothing but a pocket of restance to the north; they struggle the entire 700 page book, and then wham, the bad guys press them with the full weight of their force and annihlate them? It's just not fun to read. So evil wins. The larger army won. What is the point?