Many Americans never leave the country because the US is already pretty vast. Put Texas on a map of Europe and traveling mostly straight (with some slight detours) from the north tip to the south tip can take you through the Netherlands, Belgium, Luxemburg, France, Germany, Switzerland, and Italy[1].
There's also oceans or large distances separating it from most other countries so going anywhere besides the Caribbean islands or Canada/Mexico is going to require an often not very cheap plane flight (probably $400/person at a minimum, likely more, and roughly 2.5x that if you're wanting to go to Asia/Australia).
Also, only about 37% of Americans have passports[2], and you can go on cruises without a passport (and used to be able to go to Canada/Mexico without one, but that's changed in the last decade), and the process can take 2-3 months and $165 in fees by itself (I just checked, I'm one of those people that still haven't gotten a passport yet...would have if the pandemic didn't happen in 2020, we were planning a trip to Canada that year), so that's a bit of a deterrent as well.
[1]: https://www.texasmonthly.com/the-daily-post/how-big-is-texas...
[2]: https://today.yougov.com/topics/travel/articles-reports/2021...