Valve, a longstanding icon in PC gaming, has never released a sequel with the number three in the title, but it has made plenty of spin-offs and DLC episodes for almost all of its major games — except for “Left 4 Dead.” Thanks in large part to Geoff Keighley's exposé “Half-Life: Alyx – The Final Hours” fans learned two major factors for why “Left 4 Dead 3” didn’t happen.