They dont!
The countryside code states "Leave Gates as you find them"
Example if the farmer is moving a herd or flock from one field to another and someones come along and closed the gate it can cause a lot of problems especially on country roads.
casue getting up in the middle of the night to go chasing cows, sheeps, horses etc from the main road and wood is no fun at all, also having to see animals be put to sleep as they got out and got knocked over isn't very pleasent.