Aside from the good middleware licensing arguments, if my memory serves me, Valve employees get to shift their work to what interests them at the time, and who

Aside from the good middleware licensing arguments, if my memory serves me, Valve employees get to shift their work to what interests them at the time, and who knows what tidbits of could-be versions of HL3, Portal 3, or other interesting but unannounced (let alone unreleased) games could be in there.