Valve is not a normal company. As far as I know they still have their fluid work structure in place where projects are dictated by what the devs themselves feel like doing and are inspired by.
Icefrog (who was the lead developer of Dota 2 - and Dota 1 for many years before that) is lead developing Deadlock as I understand it. It has his fingerprints all over it, at least. It seems enough other people at Valve liked his idea of a twist on the MOBA concept to turn it into a full project.
I feel your frustration but there isn’t really any opportunity cost lost here. It’s not that they decided to make “a game” and chose this one out of all available options. If they felt like they had enough ideas to make Half-Life 3 (or any other single player game) then they would have. It’s just that this is the game they want to make right now.