Maybe because sticking it on the roof means you don’t have to:
Take the children’s seats out and store them.
Take the pram, dog bed, rucksack child carrier, shopping bags, etc out and store them.
Sheet up the car to avoid it getting covered in mud.
Wash the sheets once they get covered in mud.
Take wheels off and then put them back on again.
Etc…..
You know you can skip most of those steps if you have kids, who will cover the interior of the car in mud or worse anyway? And if you don’t have kids, I don’t want to know why you have child seats and a pushchair in the car.