The reasons Americans don't just move to whatever country they think they'd enjoying living in the most are often the same reasons most people all over the world don't. They have a life here. Family, friends, jobs and other ties to their community that would be hard to leave behind.

Uncertainty and fear also keep people from moving their lives to new countries. Especially when they're going to a place where they don't have friends and family to help them with things like expenses and childcare.

Americans in particular are unlikely to speak anything other than English and while English is often understood to some extent many places overseas it can't be assumed to be avilable everywhere. Understanding the local language is often a requirement for getting citizenship in a country and not being fluent will be a hindrance even where it isn't explicitly required.

It's also very expensive to move. You'll need to have enough money to live somewhere and feed and clothe yourself the moment you step off the plane. You'll need to store all off your things someplace until housing can be secured and that can take a lot of time. Countries can require thousands in fees to apply for citizenship and people struggling with basics like food and housing aren't going to be able to afford that. About half the country doesn't even have a passport and the expense of getting one (along with the needed documentation) would be an additional burden.

Other countries don't necessarily want you. Immigrating is difficult. Unless you can demonstrate that you've already got someone willing to hire you (and effectively vouch that you won't be a financial burden) or you have some much needed skill or one that a native citizen cannot perform you may not be welcome. No nation is obligated to accept responsibility for you just because you want to live there.

It'd be great if everyone everywhere could just pack up and leave to start a life wherever they felt like it, but there are many reasons why that just isn't realistic. There are tens of millions of people America right now living in places without safe drinking water because of heavy metal contamination. Why do you think they don't just pack up and move to cities with drinkable water? What about the tens of millions of Americans who live within one mile of toxic waste sites that have been linked to infant deaths, cancers, and other serious long-term health issues? Why don't they just move? Forget about America, how about all those starving Africans just move to somewhere with lots of food? The world just doesn't work that way.