With Steam distribution, Valve already has a fat monetization pipe in place. While they don't need game devs to use their engine (Steam can certainlly sell UE and Unity games just as well), they also don't need to directly monetize the engine, thanks to their distribution near-monopoly. If a game using the engine is successful, Valve will get their share, adding a little engine "tax" to the already massive distribution "tax" would not make much of a difference in terms of money, but all the more in terms of user/dev acceptance. They don't need to directly monetize engine licensing and offering developers tools that are much better integrated with Steam than with other distribution options is a very natural business decision.

Also, a strong presence in the dev tools arena will certainly benefit their hardware-related projects, SteamOS/box and that whole Rift coopetition thing.