Can and "wants to" are two different things. On iPhones and iPads, there's very limited access to extensions. So anybody on those platforms can't. (For now. The EU decision may change things.)

As far as keyboard browsing, doesn't vimperator cover that?

Vimperator was abandoned when XUL add-ons were abandoned, presumably because they couldn't pull it off with WebExtensions.

Partially, the reasoning they gave back then was it was too tightly integrated with Firefox's internals. Even if WebExtensions had all the APIs needed, it would have needed nearly a complete rewrite and they weren't up to it.

Ah yeah Vimium, for Chrome, not Vimperator, is the one was thinking of.