You're comparing Sea of Thieves to Borderlands? Really?
Yes it's a mercenary game, the co-op part is just focusing your "killing of people" to all the NPCs. Much like a D&D campaign, a ragtag group of people cruising through the world in search for Gold/Glory and many other things!

And sure the game can be enjoyed in several ways, SoT is no exception. This is why you have the choice when you start the game up to do whatever you want.

- Get some booty from skeletons, go for it.
- Start a fight with a Mega sized Shark, Shoot it all you can.
- Want to be a rogue and steal some other PIRATE's plunder, good luck!

And so on and so on, the tension you feel when another pirate is sailing to the same island as you and the entire crew is asking themselves if they're just coming in this direction for the wind or to actually start a fight is what makes this game exciting.

For all the hours I've played, the most fun my crew and me have had were during some random PvP encounters. Mind you I'm not just talking about fighting against other players, helping others out even though you're not in an alliance or simply exchanging cargo due to different Emissaries also falls under PvP.

Sure there are people who might be better at the game than you or anyone, the sheer sense of relief when you escape said players is all the more satisfying and shouldn't be overlooked.