That's because despite glamming it up (probably for cash) international football is pretty irrelevant these days. It's a downwards step for the quality of the champions league or probably even the domtestic leagues some of these players play in.
I imagine it's even harder for a messi who has turned up to be surrounded by poo when week in week out he's playing at a far higher level against higher level opponents.
I find it hard to get excited about a world cup these days, I look forward to the standard and matches in the champions league far far more than standard of matches in the world cup.
Regardless, I'm sure Messi would like to win the world cup with Argentina. As would any of the competing teams. If they didn't want to, they can always retire from international football, any time they like.