It's the shape of the delivered artifact that's driven the way things are implemented in the ecosystem, not a really fundamental architecture difference.

The shape of historically delivered ARM artifacts has been embedded devices. Embedded devices usually work once in one specific configuration. The shape of historically delivered ARM Linux products is a Thing that boots and runs. This only requires a kernel that works on one single device in one single configuration.

The shape of historically delivered x86 artifacts is socketed processors that plug into a variety of motherboards with a variety of downstream hardware, and the shape of historically delivered x86 operating systems is floppies, CDs, or install media that is expected to work on any x86 machine.

As ARM moves out of this historical system, things improve; I believe that for example you could run the same aarch64 Linux kernel on Pi 2B 1.2+, 3, and 4, with either UEFI/ACPI or just different DTBs for each device, because the drivers for these devices are mainline-quality and capable of discovering the environment in which they are running at runtime.

People commonly point to ACPI+UEFI vs DeviceTree as causes for these differences, but I think this is wrong; these are symptoms, not causes, and are broadly Not The Problem. With properly constructed drivers you could load a different DTB for each device and achieve similar results as ACPI; it's just different formats (and different levels of complexity + dynamic behavior). In some ways ACPI is "superior" since it enables runtime dynamism (ie - power events or even keystrokes can trigger behavior changes) without driver knowledge, but in some ways it's worse since it's a complex bytecode system and usually full of weird bugs and edge cases, versus DTB where what you see is what you get.