People want a Forza game on PC because PC has no pseudo RPG collect-em-all racing games with a sandbox career that has hundreds of events that limit you to certain performance classes/vehicle types that you can race with cars you actually get to customize and tune.
All PC has bar from NFS Prostreet and Shift 1+2 are racing games that play like fighting games, all you get to do is race and none of them are good for long sessions unless you're addicted to spamming the same tracks in the same car or two that are viable to set new times, which is legitimitate, it's a fun way to play it but there just isn't a Forza/Gran Turismo style racer on the platform and I doubt there ever will be, but like (for example) DayZ, when somebody finally gets the brains to do it, it'll be a huge hit.
Until then you have pick-up-and play simulation racing, which is good, but it doesn't give you the fullfillment that the console racers do, unless you're a hardcore racing fan that only wants to race.