Why do rocks on other solar system bodies that have an atmosphere seem to be flat? Images taken by landers on Titan and Venus and Mars show landscapes where rocks, to me at least, are surprisingly flat. Being used to walking around in forests with roundish meter sized boulders, I'd be very surprised to find myself in a landscape of flat rocks.

Their atmospheric density range between about 100 times, half and 1/100th of Earth's atmosphere. So that can't be the main explanation. Actually, I think that the landscape of the Moon is more Earth like than that of the atmospheric worlds out there.

Is this my impression of "flatness" a real phenomenon, and if so, why and how?
I would say that your initial observation is flawed, so the question is moot.

Huygens landing site, Titan: