While you may be ok with "just for fun" if they added 1v1 without re-balancing all of the classes people would cry on the forums about certain jobs being overpowered/underpowered and honestly it's a waste of development time to add something 1-2% of the playerbase is seeking all the while upsetting the majority of the players who are vehemently against this type of thing being added. Open world PvP is acceptable only in games that were started with that premise and everyone knows what they are getting into. Shoehorning this into a mostly PvE game with majority playerbase that shies away from PvP centric games is just bad business.