When I was getting my passport, I was told that only 4% of Americans hold a passport. And not all of those are for travel reasons.
The USA is a very large country. With a heck of a lot of people. But because it is so large, there is a lot of variation across it and plenty of interesting places for citizens to visit without ever going outside the borders.
It is also quite an insular country in lots of ways - news is very much headlines on a loop rather than in-depth, domestically focussed (within the State itself, and some national level) mostly, and the small amount of international news there is tends to be focussed on US troops abroad and VERY important international news. So while we would have been hearing about wildfires in California as part of an item about hot conditions causing fires across Europe (France, Greece, Spain etc) and elsewhere (Turkey, Africa) generally a couple of weeks ago, their news will only focus on California and not mention anything outside the US.
There is a lot lower level of holidays in US work - the average is 10 days per year. Employment conditions generally are not as good as in the EU (no paid sick leave, no or very very short mat leave (less than 4 weeks), etc). So there is not the same opportunity to travel long distances for a couple of weeks and have spare time for family emergencies later in the year.
And while there are big variations in the type of people in different parts of the USA, there are many parts where people are happy living their entire lives in relatively confined communities and not travelling at all outside of their local area. I'm not necessarily talking about communes or the like - but communities in the mid-west where small town life is comfortable and people are sufficiently happy, don't have a lot of spare cash, and don't see a need to get out beyond their everyday limits - it is their normal. Whereas there are others in other parts (particularly around big cities, tend to be more affluent areas, tend to be more highly educated to University level and have professional jobs etc) who would see travel right across the US and internationally as something that would be good to do. I know that's a sweeping generalisation - people in small town mid-west states do also travel and also move away, and some living in big cities never leave their own small neighbourhoods within that city.
I'll also point out that I live in a country smaller than the UK and, other than travelling here from the US when I was less than a year, I didn't travel internationally again until I was 25 and I was not deprived. We had summer holidays every year growing up, just here in this country (and I have holidayed here quite a few years since then also).
But I have also been to the far side of the planet (USA, Canada, Cuba, China, etc) for holidays as well as plenty across the UK and rest of Europe, and a fair amount of travel for work both within Europe and further afield.
I hope at some stage to go further afield within the USA and Canada as there are so many interesting places to see there (we have been a few times but US has been along the East coast only, only passing through the middle in transit, but along the length of that coast from Cape Cod down to Florida Keys).