I haven't made mods for it yet, but there are people that go down that rabbit hole too. Quite a bargain for the $30 I paid.
When I first bought KSP, I had no idea you could even get into orbit. So many different levels of accomplishments to strive for, especially now with career mode.
This game has it all - you need to learn being precise (targeted landings if you want to build a structure somewhere or picked up kerbonauts you crashed the previous time), you need to plan and do logistics (hello fuel shipments), and you need patience (rendezvous and docking). And, as OP said, you quickly end up learning a lot about orbital mechanics, rocketry and space exploration. You don't need to know the maths, but you'll end up learning it anyway, because formulas are the easiest way to compress your intuition about "why bigger ships need helluva bigger rockets" or "why when I go faster I go slower".
Oh, and a fun thing I observed watching the community - because KSP runs on Unity 4, which has an unusable 64bit version for Windows, and the game itself starts quickly eating your RAM when you go for installing part or visual mods to the point of crashing the 32bit version, a lot of people install Linux for the first time in their lives just to play a working 64bit version of KSP. Yay for more education :).
For those who don't know KSP mods, RemoteTech adds the requirement to maintain communication links (via powered antennas) in order to be able to control unmanned vehicles. You can create relays as well (usually the first thing you do is to launch a geostationary comm network over the home planet), and the mod can account for the speed-of-light delay if you so choose.
Combine it with bunch of other mods like Kerbal Attachment System (attaching and detaching parts by kerbonauts during EVA, including pipes that let you transfer resources without docking) and one of many life support mods, and suddenly you really have to think a lot about logistics in game.
I challenge you to show me a way in which video games can be optimally good. Not just better than some carefully selected bad alternative, but better than any alternative. As in, actually a good thing to do of all options available.