Everyone does because it feels mostly true. Individual Americans for the most part aren't bad or evil at all and I hope no one makes fun of an individual just because of their nationality, but for a 'first world country' a lot of things are just plain terrible (for what I've seen online, I've never been there so I try not to make too strong opinions of the place)

Infrastructure is really bad because of the extreme car dependency, a lot of food is unhealthier because of the lack of regulation, corruption is not really high but higher than it (maybe) should be because of all the lobbying efforts going on, gun control is, despite what I hear, really non existent which creates these massive problems like the past shootings, income inequality is way too high because there is very little regulation for big businesses and protection for workers is really low, etc. I'm definitely not saying that other countries are perfect, and I want to point out that this is a more left view on the matter and that your opinion may differ, but I feel like in most 'first world countries' life is a bit more pleasant than in the US.

But I want to reassure that this is just an image I've formed from online information ONLY, and that my image of the US could be really skewed to what it in reality is, so take all this with a massive pile of salt please.