Because a carpark is for parking....
The act of leaving a vehicle, usually unattended, for a period of time.
A campsite is for camping.
The act of parking a vehicle with the intention of living and sleeping in it for a period of time.
Or... Because the government says so.
if you park your car and sleep in it , is that differentMore accurately, because the applicable legislation says so and there is not a sufficient body of opinion in existence which requires it changing.