Why isn't a big AAA publisher / studio making a better 7 Days to Die?

So apparently 7 Days to Die is a pretty big hit (18 million sold) and it has some pretty big issues. I think the concept for the game is awesome but the execution leaves a bit to be desired.

That said, why isn't some big, well funded, 140 person studio just taking this concept and brining it to the masses? I feel like this game type could sell 30+ million copies if it ran better, had better art, had more ambitious scope etc...

Does anyone have any idea why a company like Blizzard or Ubisoft isn't taking a crack at this game type?