Well, the US is a pretty young country. It was the last continent to be settled (by Europeans) and the history of (European) civilization here is just not very long.
The US does have prestigious organizations from early in its history, but that history is still pretty recent in comparison to Europe. All things considered, the US has a pretty good amount of prestigious history and accomplishments compared to classical European history. It's just hard to compare 18th century America to 5th century Italy. For as long as the USA has been a country, it's not significantly less accomplished than any historic European nation of the same age.
The US does have prestigious organizations from early in its history, but that history is still pretty recent in comparison to Europe. All things considered, the US has a pretty good amount of prestigious history and accomplishments compared to classical European history. It's just hard to compare 18th century America to 5th century Italy. For as long as the USA has been a country, it's not significantly less accomplished than any historic European nation of the same age.