What you say is somewhat true. The other way to look at it is what are the alternatives? They can only fill the shelves with what they can get.
In your post you said you wanted to spend a ton of company money on a large group of GM branded cars. Not that it is a bad thing but expand on how you could save money, Like use the same engine tooling on the Chebby brand as they are the same. One small mold only paying for the tooling cost and getting more bang for your buck. If it gets a big block throw that sprue in. If it gets the small block throw that one in. Exhaust and intake could have many options on the same "molded part". Wheels could be dealt with the same way. Just put the correct wheel covers on the body sprue. Why do they constantly repay for what they already have. A smaller machine takes up less space and square footage is expensive.