Err.... I'm sorry to demolish your idea of the word and your metaphors... But there are people which we tend to call "car enthusiasts" that will do exactly that. Build car from scratch, exactly that. There's a wide range of cars available, sure. But it does not mean that this wide range is good nor that it fits your needs.
I also don't see the point in coming here to simply rant about it. As Kryptos mentionned, AI is a subset and a side-feature. If I may say, it didn't came right away in UE4 in the form of the behavior trees and I doubt it came straight away with Unity either.
Now, sorry to be blunt, but you've just gotten used to everything being done, pre-baked, pre-eaten for you and apparently are not capable of thinking about low-level problems... And THAT is my issue with it. You can deal with it and look at different solutions for implementations of AIs instead of whining.
I can't call demolishing the hard work of people criticism, sorry, it's just too easy of an escape route... "Oh sorry. I didn't mean to offend you, it was just criticism, take it and make your thing better based on my single-minded opinion".
Also. UE4 and Unity built themselves around that community. Have you seen the vastness of the UE4 answer hub and the Unity Forums?
Look.. I googled for less than 5 minutes I found documentation that could help you make an AI system AND as well for you to learn how it even works underneath (it can only get you to make a better use of it)
Intel - Designing Artificial Intelligence for Games (Part 1)
My apologize if this post is a bit rough.