It's the classical apple knows best and decides the choices or lack of for you instead of letting people who know what they're doing have access to more advanced system options.
So many aspects of the Mx devices are locked down. Not only do they lock 48khz audio on hdmi, but there is also a software block that makes all usb-c/thunderbolt/displayport alt mode adapters that work with hdmi 2.1 (with DSC) to output full 4k120 4:4:4 10-bit on windows devices get locked to 4k60 8bit 4:4:4 or 4:2:2 10-bit HDR in macos.
My M1 mac mini can do 4k144hz with both usb-c to usb-c or usb-c to displayport to a monitor so it's not about the pipeline or framebuffer etc. Not only that, but adjusting system gamma without editing icc profiles manually, video format like YUV/RGB etc, or even EDID overrides are simply not possible and as a preferred platform for music production and photo/video production by a large portion of the industry i just don't get why they can't give users more control.