Honestly it’s mostly because AFR is actually a really bad way of describing it. That slider is more accurately “How agressively do you want to enrich stuff to try and always get the best power/response, or lean for the best economy”
It does represent a much more dynamic system in the case of EFI cars, and that’s kind of simulated by the much better power/economy compromise you can get with them in game.
I think if we designed it again we’d come up with something more sensible than AFR for that slider, but it works reasonably well for now, if maybe in need of some balance tweaks with regards to its effect on final car stats 