Because they're massive SoCs with tons of onboard peripherals that aren't hooked up through some bus with discovery, like PCIE. You need tons of (often custom)

Because they're massive SoCs with tons of onboard peripherals that aren't hooked up through some bus with discovery, like PCIE. You need tons of (often custom) drivers for those peripherals + device tree map to tell the kernel how to hook up all those drivers.