When calling for reservations, we've always asked about sites and good spots. When we camped with young kids, we always asked for sites near (or next to!) swingsets, pools, etc. Memorial Day weekend, we camped with a rowdy group right in the middle of everything (YES, we were with THAT group!). Now that it is usually just wife and I, we scour campground maps for those out of the way sites, NOT next to pathways for the bathrooms.
Even with state parks that use Reserve America online reservations, we will still call the Ranger office and ask them about sites. At some of the lesser used parks, the Rangers are bored to tears and LOVE when people call! lol
Now, having said all that, in answer to the original posters question - "why do campgrounds assign sites that way?" ----- I have no idea. I would go with kcmoedoe's post, seems to cover it.
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2021 F150 2.7
2004 21' Forest River Surveyor