Because only 99% of aircraft will comply. The remaining one will be on its way to a repetition of the Washington collision.
On a STAR and approach with multiple stepdown altitudes that a constant flow of arriving aircraft is fed into, it is simply not feasible to ask every aircraft about their intention to keep its upcoming altitude constraint. This would completely clog up the frequency. Instead, having aircraft show their next intended level off altitude via their FCU window and Mode S transponder is a low-workload method to ensure everyone has the right picture. If some traffic was to proceed to a waypoint with a constraint of 5000´+ while showing a FCU altitude of 2300´ already, questions will be asked - there might even be an automated altitude alert showing up on the controllers screen.
In some airspaces, managed descent is not the best of ideas.