Well some people do care about it. It has been the key part of the game four times in a row, and you can't expect people to simply forget about it and jump right into the online mode, being satisfied with any sort of rewards/unlockables being handed to them on a platter with 0% completion.
Because it takes away much from the challenge, or whatever you want to call it, of getting 100% completion in the simulation mode. Consider the fact that some events offer more cash rewards than others. If you really, really want to unlock a certain vehicle that costs, for example, 4 million credits, what would you be more motivated to do, continue the quest to complete the simulation mode in its entirety or just replay the same event over and over again, farming credits until you have every single car in the game and leaving more than half the simulation events incomplete? The prize cars in the older games made the game exciting because the rewards for simulation mode events were unique. Simply receiving credits and a couple additional percents on the status screen is not unique. The prize car system is the main reason why I kept to the GT series when Forza became successful and I really don't see myself being interested in the series if PP only keep watering down simulation modes in subsequent games.