The rugby tackle is almost a martial art in it's own right, wearing longer shorts would give the opposition more to grab hold of, who would want to help the opposition ?
https://www.bing.com/images/search?q...ckle&FORM=IGRE
They also make special line-out shorts for the jumpers so they can be lifted by their own team to catch the ball at line-outs.
https://www.bing.com/images/search?q...&FO RM=IQFRBA