Yeah, it probably comes down to resistance. When you start to move the stick from the center, you have to work against a tiny bit of resistance. With a touchpad, there is absolutely no resistance to let you know when you are not at the center. As you say, that little rumble you get is not equivalent at all. Possibly an even more important type of resistance is how a joystick basically has its own physical boundary to limit how far you can push the stick in any direction. You can flick the stick as hard/fast as you want, and it will always stop at a certain point. On a touchpad, you don't have that resistance, meaning you have to manually slow down and stop your finger as you reach the edge, or else it will just fly off the touchpad.