I think it's a number of reasons. Number one, most Americans probably feel incredibly detached from Europe. I think something like more than half of Americans haven't even left the country? It's expensive to go to Europe. You don't know the people, the countries, it's easier to detach yourself from their livelihood and "cheer on any economic or social disruption". It likely doesn't directly affect anyone they know.
Two, I think it has a lot to do with media and political views. Anytime we've seen political debates about health care, you always hear about how evil socialism is and why universal health care is evil communism. Well, that's how most of Europe works. If people think it's borderline communism, then they think most of Europe are communists. I'm sure mainstream media has pushed out a lot worse sentiment about Europeans too. How often do you hear of Europe spoken of in a good light in media? In the news it's more often than not "here's a European country's leader who said something shitty about America", or "here's a European country who just had something really shitty happen to them".
I also think there's more butthurt than most people act about how Europeans say Americans are fat and uneducated. It's... pretty fucking rude either way. I can't disagree with that sentiment. Even if there's more obesity here and maybe our education sucks, who likes hearing that stereotype? Then there are those who say "I don't dislike American people, I just dislike your politics". How would anyone react who feels patriotic? Lots of our people are nationalistic, and hearing that they don't like our nation... that's going to offend a lot of people regardless of the reasoning. It's basically saying "your government sucks". I think a lot of Americans feel like they're the only ones that are allowed to say our government sucks.
Many Europeans express a lot of dislike for America and Americans. It's either something our country did, or something shitty about our people like our education sucking. Maybe they express that they're pissed off about the NSA. Maybe it's about the US in the Middle East. Maybe it's simply because we elected Trump. Maybe it's just because they speak our language but we don't speak theirs.
I don't know, but I've certainly heard a hell of a lot of negativity about the US coming from Europeans. A lot of it is fair. However, no one likes hearing about how their country sucks.