I said earlier, as did another poster, because we choose to arrive in France after all campsites have shut for the night. Aire de la Baie de Somme (and others) have dedicated caravan areas, with no pungent smell of urine, no refrigerated units, and are regularly patrolled by the Gendarmes - so we choose to stay there because we'd have to alter our journey times otherwise. Others, as said, choose to overnight en-route because they don't want to be tied to site arrival and departure times.
Also when the children were small we found that leaving a site in the evening, (no problem on most sites in France) after showers and supper, meant that we could make good headway, sleep somewhere en-route, and arrive on another site mid-morning - giving us the feeling that we'd had a whole extra day.
It is a choice - and if you're not comfortable with it, for any reason, then don't do it.