Because it has no well optimized servers.By the way since only 2 of you are figthing a battle the players would be only the captains.The crew would be NPC, but as in MB, it would be restrained to several groups like sharpshooters,infantry,cannoneers,sailors(melee).It could have a better made order system and also have restrictions.And to Mr.Henry, I think that would be feasibly imposibble to do a ''drive by'' with your ships just to shoot down the enemy captain.Muskeets should be made as they were, inacurrate even in 40meters.And for the sake of optimization this type of combat should be only appearing when you hooked the enemy ship and boarded it I mean, you cant just shoot with your captain to the other ship if its not near your ship and hooked. I want a realistic but an exciting game also!
For example: 80% of your crew would appear only in the boarding
So it wont affect general gameplay.
2nd idea is that if the upper one is not possible,Maybe have an instance(with the same ship you have) where you can fight the battle beetween you and your enemy i mean the other guy who was boarded or boarded you.But, the ships wich are hooked to eachother in the open-world would still remain there open to damage, and if your ships get damaged you would get the notification.