I'll probably get some hate for this for stereotyping and generalizing, but that's fine it's just my opinion anyway.
American born players grow up watching the American sports culture (NFL, NBA, and to a lesser extent the MLB) where the individual is king. The team and the idea of the team is secondary to the individuals "legacy" in that sport. There is the notion that these athletes need to create their own "brand" and whatever team they are currently playing on is irrelevant. that then carries over into the games themselves and the idea of selling out one's body or "taking one for the team" gets pushed down and replaced with "I need to win, to establish my legacy" ....An I need to win as opposed to WE need to win line of thinking, so to speak.
The idiom "there's no I in team" does not resonate with a lot the American athletes because they are praised more so for individual accomplishments than team accomplishments. Whereas in Canadian sports (mainly hockey, obviously) the team winning is viewed as the ultimate goal, and creating ones' legacy is accomplished through team success. How many people on here have stated multiple times that if McDavid doesn't win a Cup he can't be considered an all time great?
Now, I get that there are outliers on both sides of the border and this is not a truism for all American or all Canadian athletes in team sports, but the individualism does seem to be much more prevalent in the American sports culture than the Canadian.
Again this totally just my opinion with absolutely nothing to back it up other than my own personal observances, but it is what I see when I look at these 2 countries sports cultures.