With cell phones the higher frequency bands have a harder time working inside buildings and such. The signals don't have enough punch to get thru obstacles like walls. WiFi also runs in bands that are high frequency - 2.5 and 5GHz. I'm guessing that most campground operators probably don't bother sticking an antenna outside facing where the campers park. They probably just set up the WiFi unit close to their computer and figure, well it works here so it must be OK for the campers.
When I first set up my WiFi at home it ran on a computer in a back bedroom and I was lucky to even see it in other rooms in the house, and outside there was nothing. I bought a cheap WiFi antenna with a 15' cable and mounted the antenna on the ceiling near the center of the house and now I get good reception except outside where it's iffy.