Michael Cera is awesome. Half the comediens out there play the same role each time. It's because they are funny. I want to laugh when I'm watching a comedy. I don't care if it's the same way I laughed in his last comedy. I don't know why Cera gets singled out every time stuff like this is brought up.
I think the difference stems from their particular genres. When people see a Jason Statham film (I could just be speaking for myself here, I'll admit that), they aren't expecting to be blown away by an actor's "range." What I'm struggling to convey is that we just have lower expectations for what an action movie should be because the genre lends itself to a certain campiness.
Michael Cera tends to play more dramatic roles but it's always a similar sort of character, so people give him crap for not stretching further. People just expect a little more out of an actor who has placed himself in a more dramatic niche as opposed to what Jason Statham has done.