But ARM has PCI, including it's enumeration. For the many other devices (timer, uart, I2c, PCI controller itself) no enumeration is possible - you can't enumerate searching for a timer without having a working timer - only a hardware description stored somewhere is possible. The device tree is the most logical, easy to understand, fixable, updateable and extendable way to describe hardware. It doesn't have executable code like ACPI does, and that's also one of the good things.

Let's take an example. Raspberry Pi doesn't have a RTC, but it has GPIO header. You add a RTC module on that header, one of several models of RTCs.

With the device tree, you load an overlay with some parameters and a kernel driver module. And it works.

How do you do that with ACPI? Ask the manufacturer for a UEFI update that scans for dozens of RTC types on each I2c bus? Good luck with that! What happens 5 years later when the board is long abandoned (not Raspberry's case, but think of an ordinary chinese manufacturer)?