Fault is divided, not awarded
Arizona apportions responsibility among parties rather than assigning it to one. In a chain reaction every driver may carry a share, including you, and your recovery is reduced accordingly. A driver who was entirely blameless in their own account can still end up bearing part of the loss.
These claims take a long time
With many vehicles, many insurers and sometimes litigation, apportionment is slow. Meanwhile the car needs repairing and bills arrive. Your own collision cover and medical payments operate immediately and recover afterwards, which is the practical reason to carry both.
Several at-fault parties can mean several underinsured ones
When liability is spread across drivers whose limits are modest, the total available can fall well short of the total damage. Underinsured motorist cover is what fills that, and it is exactly the scenario it exists for.