I decided to answer this as an American who actually moved to the Netherlands. The other posters are Americans living Stateside who haven’t emigrated and are speculating.

There are three main reasons I left the US:

The US has a cruel social system. From its credit ratings to expensive and sporadic health care, Americans are punished for poverty and rewarded for greed. This is on top of more virulent racism, police brutality, religious fundamentalism and other signs of an unequal and failing state. There are better systems that make it possible to live free and live well. The Netherlands happened to be where I ended up with a job after having lived abroad a while. I discovered quickly how much less stressful it is to live, work, buy a house, pay for health care, etc. Things are not perfect in the Netherlands, but there is less inequality and more opportunities for educated people.

Americans enjoy being cruel to one another. They participate in a cruel system, and this rubs off on how they interact with each other and "the other." I got tired of it. It’s easier to be kind and charitable for good effect instead of satisfying one’s ego or dolling out "charity" to those considered "deserving" (almost always with flawed judgment, emphasis on judgment. Americans are very judgmental compared to many other countries.)

Lastly, I was exhausted by Americans’ ignorance of the rest of the world. The constant "American Dream" propaganda, being inundated with the country’s "greatness" despite its flaws being glaringly obvious, and otherwise being unable to see past America to other, more successful countries. The US takes that ignorance unto the international stage through its military and diplomatic thuggery, and that hadn’t done much to help the world.

I just didn’t want to be associated with America’s problem nor promote its "greatness." It’s a right-wing country with ego problems, even if its people are from many different opinions and walks of life.