He's underrated in America as an athlete still. This guy deserves to be mentioned in the same breath as Michael Jordan for his on field performances. He really is that impressive.
There's something about him that is amazing. You don't even need to understand soccer deeply to see that. Like Jordan, he has this extraterrestial/beyond human quality to him in that he's able to do things that defy logic and that other players just can't do.
For those who are knowledgable about soccer, he's clearly the best ever player at this point and like Jordan not only is he the best player to ever play, he's also arguably the most exciting player to ever play. Sports is about entertainment at the end of the day and that's hard to do.
He's not as formidable as an athlete as MJ was though. MJ was a 10/10 in every major category. Messi is a 10/10 in like 90% of major categories but he's an average defender and his mentality and leadership is also questionable. And there's the off the court success stuff which obviously he can't compete with Jordan in (not that any other athlete can). But as a singular talent, Messi is unbelievable.
I consider him the 2nd best athlete of all time because he is the best ever player at the most popular sport and also arguably the most entertaining to watch.