It's not a firmware partition, it's an interface to the nvram variables provided by the firmware. There's a bunch of reasons why these should be writable - you need to be able to modify some of them to do things like reboot into the firmware settings UI, they're used for secure boot key management, you want to be able to choose which OS to boot into on the next reboot or which network interface to PXE off, that kind of thing.

As for why userspace couldn't remount it itself - it could. It doesn't. Changing the behaviour of systemd without changing the behaviour of the rest of userspace would result in userspace being broken, and making that kind of incompatible change is annoying - especially when fixing it in the kernel allows us to avoid that breakage.