Because if it became clear that one unit was better in most situations because its DNA was not in balance with its abilities for better or worse, that unit would either not be used in the game or used exclusively at the detriment of other established units. This is what is called glitching in games and in these situations it breaks the game.
Examples ( these are cases where one unit had a clear advantage over other units and it was merely a task of buying more of these units than another player):
Tanks in AAR and AAE 3-2-2-5 or 3-3-2-5 tanks
In conquest of the empire ( original version) the side who bought up all the catapults won the game
In Napoleon in Europe, you bought is remember cavalry and it was a clear advantage in combat.
In MB AA edition , getting heavy bombers first broke the game, before that having infantry bought every turn made the game static but kept you in the game longer
IN NOVA games edition, getting a Chinese factory to place US land forces made the game broken. US would build up before Japan can take and Japan would never get going in Asia.