I do electronics professionally. This is likely because in order to save a bit on BOM costs, the device itself doesn't have the necessary pulldown resistors to signal to turn on VBUS.
USB-A has always-on VBUS while USB-C doesn't. Because the spec allows for always-on VBUS in a USB-A to USB-C cable, some devices just assume that they're always being powered by one of those cables.
I doubt it’s even saving cost, it’s just incompetence. It works with the USB A to C cable, so job done. They’ve probably never plugged it into a PD charger, and probably just copied the schematic from the last (defective) product and will copy and paste it into the next one too.
USB-A has always-on VBUS while USB-C doesn't. Because the spec allows for always-on VBUS in a USB-A to USB-C cable, some devices just assume that they're always being powered by one of those cables.