Critics review a movie against its own genre. And for the 'blockbuster superhero' genre The Avengers movies are generally pretty good. Not everything needs to be deep, a bit of occasional shallow fun can be just as fine :)
Well, I'll agree with your statement, if you agree with mine:
Different people have different opinions, and you can't make an objective claim of fact about something that is purely subjective.
You obviously disagree. What now?
We seem to have reached an impasse.
And they're not "bad", says hundreds of critics and millions of moviegoers.