Simply put because its aimed at those after a simulation racer, not those after an arcade racer.
Most serious sim racing titles don't feature restart options, forcing the player to maintain as much concentration as they can. Many people, myself included, believe that restart options on sims defeat much of the point of it being a sim. Get a corner wrong and simply restart is an easy way to advance by trial and error (and the rewind function on GRiD being the very worst example of this idea), rather than learning the hard way from your mistakes.
It doesn't suit everyone and not everyone will like the idea, but that's part of the reason why it is missing (and I would not want to see it). If you think that FC is cruel in that regard then consider Richard Burns Rally, that had an even more realistic physics engine and very realistic damage, you could drive in P1 for 5 miles down a stage, clip a curb 100m from the finish and end up with an un-drivable car. P1 to DNF within sight of the finish gate, now that was cruel, but boy did it make you focus.
Regards
Scaff