Apparently they've had one in development on and off forever, same with HL3. They just can't ever get around to finishing it before they give up, wait a couple years then try again.

Valve is like a renaissance artist with a chopped off frontal lobe. They can and sometimes do make great stuff, but are too stupid to actually finish and release jack shit.
(They get away with it because Steam prints money for no work put in.)

"They just can't ever get around to finishing it before they give up, wait a couple years then try again."

TIL I was actually a valve employee all along