Because that's how the game was designed? That's like saying you always pick rock in 'rock-paper-scissors'. Sure it's a good plan when nobody knows you pick rock, but that's not how it works in T11. Your rock might be strong enough to beat everything else up to this point (and likely will stay so for a while cause there are 10 million players, so chances are you won't meet many players who care), but at some point you might meet that paper who is slightly weaker or equal/greater to your rock and you *will* lose (troll mourinho withstanding).
Personally I want to win my matches with full dominance where possible, so I often adjust my formation to somewhat counter the opponent. Usually in the form of simple choices such as playing with a DMC or AMC depending on the opponent's strength/weakness and using arrows.
Nothing wrong with either way of playing the game, but adjusting tactics makes the game a lot more dynamic and interesting for me.