Horses are quite capable of jumping their own height, so why do they stay in fenced paddocks?
There are several reasons. The biggest is that, for the most part, horses simply can't be bothered to jump the fence to get out. (The exception is horses that seriously love to jump, and they often have to be kept in by putting electric wires in a strategic location to teach them a lesson when they try to jump out). They will, though, run out of an open gate.
Most horses, though, are more inclined to stay put as long as they have water, company, and enough food. Why bother going looking for better? (They will, though, often lean over the fence because for some reason the grass outside HAS to be tastier, right?)