In the eastern part of the country, some NF campgrounds are concessioned to a nearby resort business. They often do not have a host at the campground, but they visit the campground several times a day, and are usually just a mile or two away.
As others have said, the busier, more developed NF campgrounds often have a host, the more remote, smaller, less developed campgrounds often do not - it is pretty simple to explain. Most hosts are there for an extended time period and things like electricity and running water are important to them. If the campground doesn't have those amenities, it is much harder for the USFS to get hosts for those campgrounds. This is experience speaking as I managed the recreation program of a national forest with 30+ campgrounds - some concessioned, some USFS run, some with hosts, others without.