The size of the US is massive. Also, Americans generally don't get nearly the same amount of vacation time that our European counterparts get to take off work (standard is 2 weeks). Plus, it's also expensive to travel outside the country to the European side or even Australia/NZ.
I’ve spoken to a few Americans about travelling/holidays (vacation) and a lot of them don’t even have a passport
Why is this? 90% of Europeans have a passport