One of the selling points is performance. Especially on embedded devices userspace is not okay.
Most high performance packet forwarding is done in user space!
Check out VPP, DPDK and OFP
Well, that's cheating in the sense that it's accompanied by drivers allowing you to bypass the kernel stack and write a tailored userspace processing application.
It doesn't hold for a naive userspace application (such as the Golang Wireguard implementation). I'm sure you could make it fly in userspace using DPDK, but that's besides the point