First, immigration laws of the country one is moving to. If one is not able to get a passport from an EU or EEA county based on ancestry, you basically need to be sponsored for a work visa by a company in the country you want to move to, which can be quite difficult. And even then, you have to be employed in that country for long enough to qualify for permanent residency, then citizenship, which can take up to 7 or 8 years in some countries.

If one is lucky enough to have parents or grandparents who emigrated to the US from a European country and can claim citizenship based on that, it's a lot of work to get all of the paperwork together and verified and accepted by that government's consulate (at least it is for Germany, but German bureaucracy is ... special).

Second, the US is one of the only countries in the world that double taxes its citizens. If someone was born in the United States, they will have to file taxes reporting income to the US government every single year until they die, and PAY taxes to the US government on any income over a certain amount every year until they die, regardless of the source of that income, and regardless of the fact that taxes on the same income need to be paid to the host country.

While I have zero respect for the snivelling shitgibbon name Boris Johnson, he was born in New York and had to renounce his US citizenship to escape the IRS. You also have to PAY the US government $2350 (in cash) for the privilege of giving up your citizenship, which is also...unique.

Sometimes there are tax treaties that can take most of the sting out of the double taxation issue (Norway's is decent for US citizens), but it depends on the country.

Finally, it just never occurs to many Americans that leaving is even a possibility.