What "they know" is what you do not. You do not know that "most people only use a few cars" even though you claim to. I can only speak for my group when I say many of us try and use as many of the cars in the game as possible.
What I do know is that to me FM5 seems a much less attractive game than FM4 with only 200 cars and 14 tracks. It's a shame as I was seriously considering getting an xbone just for fm5 but that along with the ludicrous lack of compatibility with wheels makes it a no go for me. Forza could have had me, but thanks to an over strict upgrade policy and awful decisions by X box bosses, they haven't.
All of the standard cars that people want removed from gt6 can effectively be removed by the user from the game using the stockyard part of the garage.