why so so many people who are righthanded swing a bat left handed??/

I haven't seen that many right-handed cricketers playing left handed but there is a clear advantage if you can. Most bowlers are right-handed and bowling to a left-handed player is different and therefore likely to be slightly harder, also the fielding side must adjust their fielding pattern. All in all, the readjustments necessary for a left-handed batsman can add up to a slight advantage for the batting side. The same cannot be said for golf, which is much more dependent on eye and accurate aim and doesn't involve the other players. Most right-handers have a naturally stronger left eye and that is much more important in golf than it is in cricket.