It doesn't for it was released on consoles.
If you got 6-8 hours out of the game, then you might as well have beat it. That's about how long it takes (closer to 8-10).
The co-op definitely offers more opportunity for nonsensical fun, too.
But if you never got to meet the Space core... then hell, you missed out.
SPAAAAAAAAAAAAAAAAAACE!
When does it get cheaper? It's still full price on Steam.