The mainline kernel is missing the required bcm2835_codec.ko module
hardware decoding of h264 video by the Pi's Broadcom GPU requires that a kernel module bcm2835_codec.ko is present. This is maintained by Raspberry Pi at
https://github.com/raspberrypi/linux/tree/rpi-5.18.y/drivers/staging/vc04_services, but is not yet in the mainline Linux kernel (some other bcm2835 drivers already are there, maybe it will eventually be added), so it should have been added as a patch to kernels intended for the Raspberry Pi. This is done by Raspberry Pi OS, Ubuntu, and Manjaro distributions for the Pi.
It is absent in the archlinux image for R Pi.