Because I think you are very, very wrong. Because I think any given scene in Avengers is better than the entire Transformers trilogy, minus the first half hour of the first one, and that's being generous.
Because to me It's like comparing a community theater production of Hamlet with Laurence Olivier's. It's like comparing a kid tossing around a baseball with his dad to Nolan Ryan. It's just... not comparable at all.
I never did and never will give a frig about bumblebee, because he's a stupid character, like all the characters in those movies, who barely deserve to be called characters, who are never given anything interesting to do or say, who never grow, who never change, who never say anything witty or smart or even remotely interesting, who exist either to scream or fight (incomprensibly), or make awful hacky jokes, or say something racist or sexist, ad nauseum.
Oh, and those must be some good cartoons you've been watching.