Why assume that handwriting wouldn't be learned if it's not taught? How many of us took typing classes?
Because there are relatively poor alternatives to it. Think of it this way -- if people were used to T9 interfaces on phones, and you had a T9 pad for a computer, then most programmers would be entering their code using T9 because it's harder to learn to type properly.
Sure, nobody 'needs' handwriting when they're in front of a computer keyboard but its a lot easier when you're outdoors and need to write a quick note down (and no, T9 is not the answer), especially given that paper doesn't disappear when it loses charge.