While some of it comes down to personal opinion, people have many valid complaints about these movies.
It could be the awful writing, screenplay and direction, the blatant racism and racial stereotypes prevalent in characters such as Skids and Mudflap (I don't even want to remember their names, but damnit I do), the piss-poor acting and juvenile humor (dogs fucking and Sam's mom talking to him about masturbation? Bumblebee 'peeing' on an agent? Oh my sides). Or maybe it's the fact that the Transformers themselves are often jumbled messes of metal, pistons and weapons, which become indistinguishable and chaotic during the shaky-cam action scenes, or how the movies seem to be more concerned with the frankly unlikeable human cast than the actual Transformers, amongst other things