It's not in there because it's generally a gigantic hack. For the 40 or 50 years that people have been developing device driver's the basic principles have largely not changed: you start with a clean slate ans do a reset to put the HW into a known good state. Every block of silicon ever devised comes with means to reset it's internal state.

What this is doing is telling the driver to ignore it's normal reset procedure because someone else, but specifically not you, has already done the super complex modesetting and you are to trust it implicitly.

So next year, when the sales people come up with the idea that they want to show an animation across all your DisplayPort chained monitors at boot, the guys at Apple buckle down for a month, hack that into the bootloader and then hack their kernel to recognize it, ships the same year. On Linux, for the first ten years, nobody cares because there are much larger fires to put out, then ten years are spent on a vendor and display technology agnostic VR ready boot up animation description format, ten more years pass for a critical mass of boot firmware and kernel graphics drivers to support it, then it ships with Ubuntu but every third boot has flicker or a weird rotation issue.