Why the Drivatars in forza are so awful
The drivetars are on a linear track on the racing line, they dont use the game physics, they are simple x y coordinate translations being played back like a tape recording file.
When they are in front of you, the tape is played at a higher speed. its still the same song, but played faster, hence why they turn around corners at 1000mph - its because you are on the straight still and are traveling fast, so their pre-recorded movements to go around the corner are 3x of usual.
They do this to give you a challenge, the faster you go the faster the tape plays - the difficulty setting just changes how the metaphorical tape speeds up.
On novice difficulty, the speed is reasonable and somewhat realistic to the player speeds, the higher modes are cranked up way too high
This isnt just speculation, from turn 10 themselves
"We run the simulation through thousands of laps on a per track layout, per car type basis. This builds a custom racing line, enabling the AI to drive really fast on any given track. "
Changes to Drivatar AI Coming in Forza Motorsport Update 20
They all follow one racing line - turn 10 even developed a multi-line system in motorsport which actually helps a lot with the ramming problem, looks like that didnt make it into this game due to having thousands of potential different "tracks" to drive on
You can even find this out yourself by just playing the game
Challenge any driveatar head-to-head, due to the random nature of the race it cant rely on the pre-recorded line system, the real ai actually takes over - then you can see how sloppy the real ai is, crashing into every car and obstable, driving like a grandma.
TLDR: Instead of developing the real ai to be competent, they keep sticking to this pre-recorded tape BS