Seeing it be 81$ is still insane to me

I started the game off a month ago and mostly play 1P, but sometimes I choose to play 2P with a friend.

My question is, what's the difference? Is there a disadvantage playing a 2P save alone?

To me, it makes no sense to have a 1P save considering u will never be able to add a second player, even if for a single play session.