When I first downloaded KSP long ago and started to play it, I was absolutely amazed how complex, elaborate and challenging the game is. I tried to launch some of the basic pre-built rockets and I already noticed that I will have to learn a handful of new things like attitude indicator, drag and staging to get something right. But then I discovered the spaceship hangar and was like "No way! No way this game has full fledged spaceship editor. This would be just incredible." And yes, there is a spaceship editor, where you can put engines, fuel tanks, cockpits, lighting, solar panels, heatshields and a whole lot more on your spaceship. And you immediately understand how complex it would be to build an efficient rocket and make it reach the destination.
Later, when you spend a couple of days watching tutorials and getting familiar with base controls and rocket building, you realize that you can do a whole lot more than you thought you could do in KSP. You realize that you can launch a probe to mun. You realize you can launch a spaceship to keostationary orbit. You realize you can go to other planets. Build spacestations. Do dockings. Mining resources. Capturing asteroids. Build rovers. And each time it's like a revelation and you think to yourself "This game is so much more complex than I thought it was".
Each time when you crush into surface, or you don't have enough fuel to return, you understand that that's entirely your fault, and you could avoid this by being better at the game. And you start reading about orbital mechanics, gravity drag,Tsiolkovsky rocket equation, Hohmann transfers, Bi-elliptical transfers, launch windows, gravitational maneuvers, Oberth effect, aerobreaking, orbital rendezvous, etc. Each of these things exist in real world space programs, so it's extremely educational. And each time the complexity builds up opening for you new opportunities and ways of playing. This is already enough to make the game worth playing hundreds or thousands of hours. And then you discover that tons of mods are available, some of which completely blow my mind (like Principia for example, which adds full N-body gravity to the game).
KSP devs have done a perfect job of creating an extremely complex game without overwhelming the player. You start with a few simple things you have to learn to achieve at least something, and the more you play, the more you submerge into this complexity that makes you want to learn new things about the world. There are a couple of downsides, like bugs or inconvenient interface, but overall the game is very polished. Each space mission in KSP will take you a good couple of hours or even days to get right, depending on difficulty, so this is perfect for quarantine. Highly recommended for anyone who has a lot of time to learn new things about rocket science and perform extremely exciting space missions.