The question isn't one of ease of parsing (we all know YAML isn't easy to parse :-). It is a matter of ambiguity. The !<verbatim> syntax was added very late in the game, to allow avoiding the tag prefix games.
The thinking isn't "domain" vs. "local", the thinking is "full arbitrary URI" (inside <...>) vs. "suffix added to some arbitrary URI prefix" (without the <...>). The difference between "!", "!!" and "!foo!" is just "which URI prefix should we use here".
If we said "we don't attach any prefix if what follows the "!" looks like a complete URI" we'd be entering a world of pain. URIs can be in all sort of forms: "urn:isbn:0-395-36341-1" is a URI and hence (if someone wanted) !<urn:isbn:0-395-36341-1> would be a valid (if somewhat insane) tag. So "looking like a complete URI" isn't really easy to define. "Looking like a tag URI" is well-defined, but YAML really doesn't insist on using "tag URIs", even though we call the "node type tags", well, tags.