Why to people refuse to help each other out at work?

I love my job. But there is always new equipment and technology. When I learn how to use the new things or the new system, I share what I know with my workmates. However, there is one person who knows EVERYTHING but when you ask her to show you how to do something new she pushes you out of the way and does it, then she will make nasty comments about how you don't know anything. So I muddle through, I learn things on my own. When new people start or when my coworkers need to catch up, I show them what I know we all learn eventually by helping each other, except for Ms. Stupid and Selfish.

What's up with this person who doesn't want to show anybody anything? I call it immaturity.