It's not that, it's down to how the development of the game runs. Dan from Turn10 explained it that they spend months working on the actual game and nothing but the game, then when that main part is done you suddenly have a bunch of guys sat around doing nothing. Well, they then start working on other cars with the part of the team that only does cars, and those cars are finished after the cut-off date for the actual game. They cannot then go into the main one, but can be finished in time to be put on the store as DLC.
Obviously those guys need to be paid for their time, so that's why you pay for DLC and it's not just free. I've no doubt that perhaps one or two cars could've been included in the game, but then people would complain that the DLC was too little and wouldn't have coughed up so it also makes financial sense to hold those two back.
I have no issues with DLC or MTs or anything like that in games. Sometimes I'll pay for them regardless if I'm a huge fan (Forza, Mass Effect) and sometimes I won't bother at all despite really enjoying the game (Dishonored, Dead Space). Make it worth my while and I don't mind paying for it.