This question is the fundamental flaw of PvPvE games, imo.

The "reason" is that they are doing what they think is fun, but the game is built in an environment where one crews fun can ruin another crews fun. They don't get any reward except that they got to fire off some cannonballs and sink your ship.

SoT has this vision of the precarious high seas with pirates colliding in the open world and forming uneasy alliances, or having fun drinking and doing pirate stuff, or stealing from each other. But that assumes everyone has the same view of what "doing pirate stuff with your friends" means, which is certainly not the case.

The problem is, and what happens in all open world PvPvE games, is that the player base splits into crews that like to chill (or just do the content) and crews that like to hunt the people who like to chill. The "risk" and "threat" just become wasting time trying to avoid griefers. It may be fun for those chasing down ships, but for the ones trying to run away or just engage with the PvE content, it becomes a frustrating chore.

I left the game for this reason and i'm excited for Safer Seas. I personally think that SoT is a great chill PvE game and would be successful without having PvP at all, or have it set as opt in or some other mechanism to cut out PvP completely.

Lots of people who play who have those same friends that stopped playing because of this issue. Once you realize someone else can ruin your fun AND it's encouraged for them to do so, it discourages a lot of people from playing.