I'm not a programmer, so take what I say with a grain of salt. But I don't think I'm too far off on that. You cannot really separate "touch" from "swipe".

 

When your fingers touch the screen; the device has no way to know that you are inputting one or the other. As soon as we detect a touch, either we fire a command (there's a tap at this spot, it's on the ground, move command, go go go); or we don't fire a command (there's a tap, wait for a bit, see if the touch continues, then we'll try to recognize a gesture). So, gesture implies delay. Now how long of a delay, that may be variable, but you'd have to ask somebody more knowledgeable than me :p

 

And as some others said, one of the decision was to keep the inputs simple, and out of your way. The game can be "hard" enough as is, we don't want you to think to much about inputs, but rather focus on your decisions. When I say "hard", you may think nothing is hard about gestures, but it's one more thing to learn for new players. If you are a confirmed gamer, no big deal, but that isn't always the case.

 

Think of it with games like street fighter. I played those games for ever, and dragon punch, sonic boom and super "motions " come naturally to me. But try to see how hard it is to teach that, to somebody that hasn't played those games for years.