PCs are far more functionally standardised than embedded systems. And in fact modern PCs do use something similar to, but in fact far more complicated than, DTs

PCs are far more functionally standardised than embedded systems. And in fact modern PCs do use something similar to, but in fact far more complicated than, DTs - ACPI tables which not only contain static data but also callbacks to let system firmware do some of the work. Even on a PC there are still many things that are not on discoverable buses but I2C or SPI. The only reason it seems simpler on PCs is because the ACPI tables are supplied by the hardware vendor so they're mostly invisible to the end user. But that only works because they're mass produced and the variability between different manufacturers is fairly limited. Microsoft and Intel specified what the system firmware must do and enforced it through certifcations. Most PC firmware is also only tested for the case of booting Windows and Linux often emulates bugs... The same wouldn't work for embedded systems which need far more flexibility. Yes some ARM systems these days use UEFI and ACPI but that is for server hardware which is basically a PC where the x86_64 processor has been replaced by ARM and it is desired to have it otherwise work the same.