Sim City and Sims both revolve around just managing a city/people's lives with no combat and both are popular games. Same as Tetris which is a really simple game. Flight Simulator has a ton of fans and it's just revolved around flying. Racing games are an entire genre that is only based around racing and no combat.

Not everyone thinks you have to have combat to have an interesting game. Or that the only challenge is in combat/shooting things. And most certainly many of us don't feel like we have to play against others to make it interesting (I outright don't like PvP games save racing games. The only MP I am slightly interested in is co op games and not a big fan of that either, I prefer single player period).

And this game is revolved around exploring. The combat it does have is more to give you resistance so that you have obstacles to overcome. But it's not the only thing. You have to make sure to be able to keep enough fuel on you for your gun to mine things and to keep out the cold/heat and find enough fuel to keep going. The sentinels are to add extra resistance but are definitely not the only thing there to give the game interest (and many things are not combat related at all).