Another wild guess: a sphere of gas collapsed to a rotating disk. If you take a sphere and squash it flat, the middle has most of the mass, and it declines to z

Another wild guess: a sphere of gas collapsed to a rotating disk. If you take a sphere and squash it flat, the middle has most of the mass, and it declines to zero as you get farther. I would guess the pattern of falloff of a squashed sphere is roughly logarithmic. That pattern gets quantized into planets, and so we see the original spherical distribution in our planets.