man,if my outboard broke pull ropes that often,I would have it looked at to see why it starts so hard!! lol
as others have said,either something is cutting/chaffing it,or its too short and they will not last long this way.
after you have replaced the rope,and before you re-install the cover,pull the rope through its full length and watch it to see if ANYTHING is rubbing/chaffing on it,or if it is tracking straight with the entry hole,and the pulley it is wound onto.
a lot of times what I have seen is where the rope comes through the hole that the handle rests on is worn sharp on the inside of that part of the housing.