Exactly what they said. An ideal mattress is something that has increasing density and support the deeper you sink. Inexpensive futons, of which I have owned several, completely compress almost as soon as you lie down on them and you spend the night grinding your bones against the floor. Sleeping on just a topper has similar problems (done that too).

For me the minimum seems to be about 2-4 inches of total foam in two different densities as described. For packing into a car it also means you can take the mattress apart and stuff the pieces wherever they'll fit, and it's way lighter than a futon.