It's 2025, 8 years and 8 versions after launch - what happened to Project Treble?

From my understanding Project **Treble** was supposed to separate Android's OS data from the OEM's binaries (which are more or less equivalent to the drivers one installs on Windows) into different partitions. From that point on, Android device certification would include ensuring a device could boot Google's Generic System Images and Treble support was supposed to become mandatory for devices released with Android 9.0 or later. By this logic, seven years later, everything one could reasonably get their hands on would be Treble compliant - yet custom ROMs still seem to be made bespoke to individual devices, and I do not find much mention of people installing Google's GSIs. Why? Did Treble fail in its original purpose somehow? Why is there still no equivalent to doing a clean Windows install for Android devices?