I think it's because of America's and Western Europe's history of imperialism, not to mention that America's foreign policy is highly controversial. Most nations don't like having foreign troops on their soil, unless they asked for help.

There might be other reasons, too. We have a huge population but consistently get last or near last in terms of public primary/secondary-level education, which gives us a reputation for being ignorant, uneducated, etc. Add to that the obesity problem and most other countries probably think America is filled with fat naive hillbillies, hence the stereotype.
Sad but true. That said, though, if we didn't have the biggest economy and tons of world influence, people probably wouldn't care as much as they do, tbh.