That's technically not true – they've licensed it out to Facepunch to create a sequel to Garry's Mod (which was one of the most popular Source 1 games): https://sbox.facepunch.com/about/. It is the only one we are aware of though.
I won't be surprised if there are some other licensees eventually but it seems like Valve isn't overly interested in making the engine usable in the same way that Unreal, CryEngine, Frostbite etc are.