The point is, having to stick your arm out to touch the screen over and over or for an extended period of time is an ergonomic nightmare.
The mouse is a nightmare: First you have to find it on your desk or whereever you happen to work. Then, you have to focus on the screen and try to find that tiny little arrow. Finally, you have to coordinate movement of the mouse and arrow to click somewhere.
We are very used to it now, but it's a nightmare anyhow.
I don't even start about the trackpad!
How much easier to just stick your finger out, you don't even need to divert your eye's attention from the screen. Its natural. It fast. And your and fall instantly back onto the keyboard.
Anyway, point & click scenarios can be argued. Click and drag scenarios are where touchscreens fail entirely in their current implementations. At least on my tablet, it requires me to long press, drag, long press again, and press copy. Minimum of 5 seconds with some practice to copy some text. With a mouse and a left hand on my home keys, I can copy text in under a second.
I don't need to take the eyes of the screen, but maybe I am not the everage user. If you take a look at how less tech oriented people use a PC, a touch interface would definitely be a plus, though. Its like stylus against finger.
In addition to that, using your finger is not nearly as accurate as that "tiny little arrow" (which is of adequate size), and it obstructs your view of what you're pointing at. Touch screens are only good for big, bulky, simple UI.