I can boot all other arm devices with usb? Huh, maybe I’ll get an android phone and chromebook after all.

Is there something in ARM that prevents booting from external disks? What about Raspberry Pi?

Nah. Just that most manufacturers sell the devices in such a deeply locked state that boot disks are not supported.

Though ARM appears to be missing an open-spec BIOS/Boot standard like x86 has.

SBSA-compliant systems boot with UEFI+ACPI. Server customers care about standardization, so boot was made "boring" and basically identical to x86.

It's just the embedded junk (which includes Apple) that doesn't care. Apple has vertical integration, they only care about the full experience including their own OS. Embedded SoC vendors only care about having some "BSP" fork of Linux because that is enough to make a crappy device with it. There's just, unfortunately, zero motivation for these vendors to embrace standards.

The rest of the world doesn't care about security?