Because they're not circular arcs with a constant radius. They're "squircles". Apple uses those all over the place.

https://arun.is/blog/apple-rounded-corners/

In my opinion it only makes sense for reflective surfaces (e.g. cars) to have nicely flowing reflections, while most of Apple's products have a matte finish. In 2D UI elements it most definitely makes no sense. But some designer must have been a big fan of this and somehow they now use it everywhere.