Actually, we've stayed at some very nice KOA's over the years. On a couple occasions we've stayed several days. But mostly, we use KOA's for overnight stops, don't even unhitch and move on the next morning.
I think of a KOA as a fast food restaurant... they are conveniently located, easy access, and good for a "quickie!" But when I want a good Ribeye steak and a real baked potato, I go to a real steakhouse. That stakehouse may be out of my way, but the satisfaction of really great sit-down and be served meal is worth it.
So KOA's are great for quick access, in-out service, usually not very big campsites, (that depends tough), but also priced higher that I'd like to pay.
Private campgrounds, state parks, national forests might be cheaper, but may be out-of the way, but also are great for relaxing and spending time at THAT park! That's the difference I think. Also, KOA's are designed for young families with kids and they provide entertainment opportunities for the kids. But for a quick bed-room spot ... they can't be beat!
FYI, we have the KOA app on our cell phones... comes in REAL handy sometimes!