Which is how most non-indie games are made. You make it sound like its some insane feat that no one has done before when its actually how almost all standard games are made. Usually devs look at what the specs of the game they want to make are, find a suitable engine or make one. Its why Blizzard uses Unity for Heartstone since its a relatively simple card game but uses their own engine for Overwatch which is a lot more demanding. Mount and Blade devs also used their own engine which is really good at having a lot of NPCS running at the same time, its rare to find a game engine that can handle 1000+ individual NPCs at the same time.
In the same vein as this the KSP 2 devs should have seen that KSP 2 would have benefitted from a custom game engine where they had complete control over the physics. This isn't a small production, KSP 1 has sold over 4 million copies and Private Division bought the game rights because of this. KSP 2 deserves to be made with a proper engine and not Unity again.