Because it feels good and because we get horny and need sexual relief almost every day (or more when younger).
We also jerk off more when we are alone and browse sexy websites and get aroused because when we get aroused we cant concentrate on anything else until we have jerked off and had an orgasm.
Then we feel dam good and so relaxed.
Lots of us jerk off because our wives or partners don't want sex and we do so its so easy to do it ourselves.