Crumple zones don’t crumple when hitting anything as soft as a person. I had a car run into me while stopped. They were doing about 45, it was the worst-case impact, driver corner to driver corner. My airbags didn’t go off. I lost the left front fender and headlight. No crumple zone changes (that’s part of the unit body, when it gets bent, it often totals the vehicle). A pedestrian would’ve bounced off that car with broken bones and a concussion, minimum.

They’re for occupants.

Plastic bumpers are the only thing that compresses easily enough to not injure a pedestrian. And even those are pointless, at a speed where a pedestrian impact would compress a bumper, is fast enough to transfer a lot of momentum into a human body, and compress the bumper into the harder parts of the car.