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Here is a decent article on a related topic.
Why Video Game AI does not Use Machine Learning
Forza uses machine learning, which is why its AI is decent.
But the problem is that it is expensive and difficult to make machine-learning based AI that is near-human in capabilities without making it too good, where very few humans will win against it. It's only been recently, and costs a decent amount of money (through labor costs) to get the right mix of algorithmic (via a conditional statements) AI and machine learning AI in games. I suspect that we'll see more of this by the 2030's, but right now publishers aren't seeing the advantages. It'll take a few innovative publishers/developers or some very ground-breaking middleware to push the boundaries and set the standards for everyone else to the point where they are willing to staff machine learning engineers (who make in the 100-150k USD on average and therefore aren't cheap) or not need to staff machine learning engineers because the middle-ware does the task for them.
Of course you can make game difficulty responsive to traditional AI methods, but again, that is costly as it requires more game testing.
