Nobody owns the orbit everyone depends on
Orbital slots are allocated nationally while the consequences of crowding them are global. That mismatch is the whole problem.
A shared cost, private benefit
Each new constellation is individually rational and collectively expensive. A single fragmentation event raises the risk for every operator, including ones that never launched anything.
Disposal is the weak link
End-of-life deorbiting is improving but remains voluntary in practice, and the satellites already up there were not designed with it in mind.