The fact that the hair matches Kaylee, does not necessarily mean that she was dead while in the car...as mentioned in the above post. It is possible that even a hair of hers (Kaylee) which was on some other item, got transfered to the trunk. After a hair falls off your head, at some point it does decompose (after awhile), but that doesn't mean you are dead.
I do believe that she has died, but we just need to step back and realize this is "probable" and not concrete evidence. They will need more than this.
I remember on one of the talk shows (either Greta or NG) Cindy actually called in herself and asked one of the experts this question. I'm sure that is why they aren't believing that Caylee is dead. If I were them, I would hold on to any possible evidence against it myself.