Probably because the entire game would be different from start to finish. Maybe Steve Blake would have been way off in a game. You are acting like you can just insert a variable into a sea of variables and they will have a similar result. It doesn't work like that.
Steve Blake was 2-12 against the Spurs and 4-11 or something against the Pelicans, both close games we could have won. With Kobe in the lineup, I'm sure we'd get easier shots throughout the game and another defender that could at least partially slow down the opposing team's player that's hot.
Obviously the entire game plan is changed. Hell, maybe we wouldn't even have a close game in the 4th quarter. But bottom line was this: If Kobe was there in the 4th quarter when we couldn't generate any offense, we most likely would have won those games.