There are no other games really like this. You will, in a terribly entertaining way, learn some fundamental of orbital mechanics, rocket science, aerodynamics, astrophysics and even a bit of mechanical engineering and straight physics.
There is nobody to shoot, no zombies, no monsters, no combat. No grand conquests and raising flags over captured enemy bases. Yet you have never and will never play a game that will challenge, entertain and reward you like KSP will. The learning curve is stiff but not terrible. You play it at your own pace because, well, there is no other pace you can play it at. Games like Dark Souls are 'hard' because you've got to have the right mix of luck and muscle memory and you have to throw yourself at it again again. KSP can be hard because calculating the right setup for the right injection burn to put you in a viable position to get past Ike into a stable orbit around Duna with enough fuel to get you home is a staggering undertaking, a lot of planning and forethough.
This is one of those games that changes gaming. You will kick yourself, repeatedly, when you look back and realize you didn't have a chance to play KSP.
It is a complex, indepth, engaging, well balanced and changing but simplified to make it fun to play simulator for building and deploying rockets and ships to go into space, visit and explore other worlds and travel a solar system.
The developer is a big part of this success. One of the first trail blazers on 'early access' they have been amazing for involving the community. Several of the top modders for KSP (they love and support modding in KSP - not to fix bugs but add content) were engaged and used in building content for the 1.0 release even. They are still engaged in improving and adding content to KSP.
I've got close to 800 hours in at this point and still haven't really played swaths of the game, there are dozens of mods I want to try but haven't had the chance. This is the kind of game that's fun and engaging for years and years. Sometimes you need to step outside the regular run of the mill and see why a game like this has tens of thousands of reviews that are almost universally positive.