The Transformers movies are an unfortunate reminder that money talks. Films are a commercial endeavor. Therefore, if a movie is successful as sequel, or another movie like it, will be put into production.
That explains how Transformers 4 got green lit. The movie, even from the sizzle reel, looks bad. Really bad. Why is there a transformer in a trench coat? Why is Optimus Prime riding a robot dragon?
Because the other films made money. It’s that simple. If you want these films to stop being made, you need to stop supporting them. It’s the only way. It’s the only thing you can do.