Why don't people use their cruise control more often?

I for one doNot use cruise control when I drive my Dodge because it doesn't have it. When I drive my van I use it when I can but it sure gets frustrating when you approach a car at a speed of 68 and speed up to 75 to p and they speed up, I guess they just want to bee in front cause when you get back behind them they slow backdown. and speed up when you try to p.