This just happened to me the other night.
I’m in a dispersed camping spot in a National Forest. It’s not crowded. I go to sleep around 9:30PM and wake up at 4:30AM to someone parked right next to me. There’s no trees between us and no possible way that they did not see my car.
As I drive out of the forest I pass dozens of empty spots.
This has happened to me twice. Who are the people who do this? There’s no rational explanation for it.
After rereading they do mention they’re at a campground. I focused on the ‘dispersed camping spot’ phrase.
The only conclusion I can come to is OP uses different camping terminology than I’m familiar with. For me ‘dispersed’ can have a broad range of meanings but always means ‘not at a campground’.
Even with dispersed camping (which is not at designated sites) you can still drive to them. We do so all the time. They’re very popular here in Colorado. We literally take a 27 foot popup camper there, my brother in law brings his toy-hauler, my in laws bring their class c. And you just drive right up and camp. There’s no marked site, it’s just dispersed.
Just yesterday I got back from a trip at a designated campsite, which was marked, and you were not allowed to camp outside of 100ft of the marker. Notably this was a bit further up the road from an actual campsite, which had rules about hours, restroom use, no equines, no dogs off leash etc. and the sites were maintained spots with gravel etc.
So there’s three levels, and dispersed is just the ‘free-est’ of them all, but it has no bearing on whether you can drive up and plop down a camper or not.