Because this is a completely opinionated matter, some people like them playable, others dont. My reasoning for this is that it takes away the badass factor from the bosses if they are playable. Look at how they made Kahn in this game, he actually reminds me of the ass kicker from MK 2 & 3, not the piece of shit he was in Deception, Armageddon and DC Universe. I get such a rush whenever I play him and I feel so accomplished whenever I beat him. However, if he was playable, I would not have that same feeling at all because in the back of my mind I know I could be using that same character. Its just a buzzkill in my opinion. Bosses are given the title "boss" so they are an obstacle to get past, not to be played as.