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.

The problem is that the satellites are moving very fast and there are a lot of them. The mean free path may be very long, but if the satellite is moving very quickly it will cover that distance fairly often. This means that they will have to actively avoid collisions fairly often.

https://en.wikipedia.org/wiki/Cross_section_(physics)

Wait, are you saying each Starlink sateline is 16 thousand square miles large?

At this point I think it's entirely expected that a capitalist corporation will gleefully ignore any and all external consequences