Cuz it wants to play.
My cat doesn't swat my fingers, but he loves to lay down between me and the laptop.
try wearing a pair of boxing gloves while you type
The same reason why mine likes to sit in my lap when i start to get up, they like to be annoying :D
she or he wants your attention my rubs against the screen to get mine :)
She's emotionally needy,and thinks that you're super.