Because it can take 5 years, in which time the product is obsolete.
But, the core idea behind SBSA and SBBR is to form a common platform to which the vendors conform their machines so they don't have to keep up-streaming special little drivers for their special little SOCs. Only time will tell if its a success, but a large part of the community has effectively declared that they aren't really going to conform to the ideals of a standard platform. So, the ball keeps rolling and new DT properties keep getting added, and new pieces of core platform IP keep showing up. At this point arm64, despite just being a few years old already looks as crufty as much older architectures with regard to GIC versions, dozens of firmware interfaces, etc due to the lack of a coherent platform/firmware isolation strategy.
But, the core idea behind SBSA and SBBR is to form a common platform to which the vendors conform their machines so they don't have to keep up-streaming special little drivers for their special little SOCs. Only time will tell if its a success, but a large part of the community has effectively declared that they aren't really going to conform to the ideals of a standard platform. So, the ball keeps rolling and new DT properties keep getting added, and new pieces of core platform IP keep showing up. At this point arm64, despite just being a few years old already looks as crufty as much older architectures with regard to GIC versions, dozens of firmware interfaces, etc due to the lack of a coherent platform/firmware isolation strategy.