Why? Have you seen all the top level players of all time, to compare and make a judgment?
If you want to argue he was the best player in the last ten years, then I'd agree he was, by a short head over CR7, but (a) no one remembers far enough back to evaluate all the players and (b) the way the game has changed makes comparison difficult.
Also, Messi has played his career in a league where there is a concentration of good teams at the top and rich pickings against the others, thus goals galore - yes, you can only beat the team in front of you, but it's a factor in judging greatness, as is World Cup performance.
Compare Messi with Mardona in that respect and the latter has a far superior record - Messi is, however, a far superior sportsman.
I've been watching football since the late 60s and would judge Pele the greatest in that period, factoring in his tremendous World Cup performances, with Maradona second and George Best in third (mainly because he didn't play in the World Cup, not because of his skill level - he was the best all round forward I have seen, when interested).
Names that crop up as 'greats' from the 40s, 50s and 60s include di Stefano, Puskas, Garrincha, John Charles, Stanley Matthews, Telmo Zarra, Valentino Mazzola, Jose Manuel Moreno, Adolfo Pedernera. I didn't see any of these people play, but the anecdotal evidence is strong.
There will be good shouts from the 20s and 30s, too.
So no, it is neither funny nor absurb to say that I am unsure if Messi is in the top 10 of all time, as it is impossible to judge that - I was reacting to the sweeping statement, not deriding Messi, who is a great player.