First, the success of VR is an existential risk for Oculus. This is not true for Valve; Valve just doesn't want to be left behind (at least at a business level, I have no doubt their engineering organization wants to continue to be the tech leader).
Second, they are very concerned about whether people have good VR experiences, because the well has been poisoned by people's memories of 90s VR, Cardboard, concerns about motion sickness, overhype, headsets looking dumb, etc.
Third, they're a fairly small organization, and they already have to support engagements with GPU vendors, OS, as well, as getting their hardware built and supported. Having to support other hardware on their platform (especially as there is a race to the bottom between HTC, Razer, etc) is a huge additional burden. They do have financial resources from Facebook, but you can't just throw money at an engineering org, you have to scale carefully.
So to make VR is successful, they have to make sure that people have good experiences, but they have a small team. Buying exclusives makes perfect sense for them: it's a way to leverage Facebook's financial resources, and since the titles are exclusive they can focus on making those titles work as well as possible on a SW/HW stack that they own.