It's a stretch to say DOTA 2 or CS:GO were initially made by outside firms. Sure they're based on community mods, but so is PUBG, TF2, all the autochess games. Outside of Half-Life and L4D (which was originally a Turtlerock game) all of Valve's games are polished commercial versions of mods or student games (Portal series), but with the exception of Counterstrike all of the originals were good ideas with fatal execution flaws.

Valve has also released DOTA Underlords and Artifact in the two years. With HLA and Underlords Valve is actually releasing two games this year.