Some part of me thinks you (and the people voting) have little clue of what building an actual video game engine is like for current day expectations..
It's been discussed by Kunos themselves as well (and as someone who has a very minor background in this stuff, I'm only in the slightest qualified), you'd be surprised that things are not as easy as you think they are. Making sure (choosing) an engine is suitable for your (niche and highly specialist) purpose can be a very difficult task, as you need to know the in and outs of the engine to judge it. Building an engine is also quite difficult, especially with changing requirements. An engine is not just "something that can do everything" but every engine has its purposes and limitations while requiring a lot of learning for the team as well.
I think the only mistake with Assetto Corsa was that some core aspects of actual racing were not thought of from the start (they only built what they needed at that point), and they probably didn't see their own success coming. This is why it brought the entire "Assetto Corsa is a driving game" while it states "Racing Simulator" on the title screen type discussion. Give NetKar Pro (their previous work) a go and see how you feel about that
That other major mistake is probably not hiring a community manager and mistakenly thinking they could deal with all the crap and critique themselves.
This Ilja guy likely also works without a deadline, in a world where he doesn't have to push out a product and already has a source of income. That, and a few other reasons, are often why some modders can achieve the impossible. That and a lot of dedication.