Thanks for the suggestion! It would be cool to use the LINK verb.
The problem is that it's way less likely to actually get adoption using the LINK verb. The discovery step adds an opportunity to add a layer of indirection, which lets us separate the server that handles the webmention. While discovery is an extra step, but at the same time it provides a lot of benefits. For example, it wouldn't be possible to have a static HTML site handle the HTTP LINK request, whereas it could delegate to a webmention service by using the link tag. Additionally, some hosting environments may not provide a way to handle the LINK request where a normal POST would work instead.
Because of the benefits and flexibility the discovery step adds, we need to stick with it and can't use the LINK verb as cool as that would be.