It's not possible to do highspeed crypto from userspace while still integrating with the usual networking stack. Tun devices result in copies and context switches which makes things quite slow.
This also adds quite a bit of simplicity.
But sure, I agree, it's scary. But not nearly as scary as the obscenely huge xfrm/ipsec layer in the kernel. Or the dozens of other kernel tunneling deivces available. And I dare even say it's not as scary as most userspace VPN solutions like OpenVPN which are massive code bases. WireGuard, in contrast, aims to be quite minimal in its codebase, so that it can be audited by an individual.
I didn't make the title of this HN post, unfortunately. But do note: there _will_ be userspace implementations of WireGuard. It's most certainly not tied to the kernel. It's just that the OP saw me present at a kernel conf about it. :P