The main thing is there's relatively little incentive to standardise: the kind of hardware you write device trees for almost always isn't the kind of hardware y

The main thing is there's relatively little incentive to standardise: the kind of hardware you write device trees for almost always isn't the kind of hardware you sell to customers to just load whatever OS they fancy on it, it's generally a special-purpose device which is intended to run one software stack. The fact that there's even a standard like devicetree is basically just the linux maintainers trying to avoid hardware companies writing a million custom initialisation routines. (AFAIK there is also some standardisation around providing a devicetree to the OS from the bootloader for ARM servers, where there is such an expectation from customers, though ACPI is also often used)