That's a fair point -- other pirate games have less ships.
I think the big difference is that the Skeleton Ships in Sea of Thieves actually have skeletons that you can fight, and there's more than one way to sink them. That adds a whole lot more gameplay/immersion than just "I shoot cannons at an empty ship."
Think about it from a PvE perspective: There's currently no such thing as raiding/boarding ships in a pirate game. If you want to swashbuckle something, you have to go on land and dig up a chest. But even that combat is clunky and unsatisfying -- which is why ATLAS combat is pretty much just a sniping game: Slowly reload gun via mini-game > Shoot once > slowly reload gun via mini-game.
It would have been cool to see a really rich move-set to allow for more detailed combat. Things like rolling, grappling, execution animations, dodges, etc, could make melee combat more engaging, and make ground combat more viable.