People say that coz their memories of Messi are still fresh.
No, they say it because they believe it to be true.
I could just as well say you think Zidane was better just because you've forgotten that he had many ups and down, struggled with consistency at Juve especially, and that you ignore that he also had brilliant players like Davids, Del Piero, Figo, and Ronaldo around him. Not to mention France incredible defence - Thuram, Desailly, Blanc and Lizarazu - shielded by the likes of Vieira, Petit and Deschamps. But I'd be wrong about that, wouldn't I? You just believe him to be better than Messi. Which is fair enough, even if I disagree.
At 24, Messi has achieved more at club level than Zidane ever did throughout his career. I'd wait a bit before claiming he can't adapt to another environment but the one at Barcelona. At the same age Zidane hadn't won a thing and was still playing in France if I'm not mistaken. Messi has plenty of time to step up at international level, if that is even necessary.