Because the orbits largely
aren't 3D. These constellations orbit at the same altitude. Altitude changes are very expensive because you can't just go "up", you have to go "forward" faster. "Down" means slowing your orbit.
Two objects at different altitudes cannot maintain the same speed and relative positions. A higher orbit necessarily must have a higher velocity than a lower orbit. If you try to distribute your constellation across multiple orbital altitudes they'll slowly drift out of sync without constant thrust.
So these constellations trace out different and intersecting paths across the surface of the same sphere, not a 3D shell around that sphere.
