I didn't find any implementation of OpenVR besides SteamVR. And SteamVR is only distributed with Steam.

I suppose nobody adopted OpenVR at all? why is it that apps purchased on Steam work with the Oculus Rift?

No one who would distribute it independently, yes. Let alone open source. Do SteamVR games work with the headset without Steam installed? Open in OpenVR is a major misnomer really.

Apparently OpenVR applications will work on the HTC Vive, without the Steam client.

Oculus only made certain games they actually funded and help make exclusive to their store. It's a business decision because why would you pay for something to be made then give another store a cut of that profit instead of your own store?

Also, check out Revive to be able to play Oculus games with your HTC Vive :) Compatibility list is HERE

So, after several weeks of having used the HTC Vive and keeping up with the VR community, a lot has happened, here are some highlights:

- There are games on GOG which have VR Support: "Mind: Path to Thalamus" and "The Solus Project" to name two; "The Vanishing of Ethan Carter" did have experimental Oculus support at one time but was removed. The Steam version still has VR support available as "additional content".
- Several VR games and demos have been released on Steam since Oculus and Vive headsets have shipped. Current count shows 240 titles: 222 titles compatible with the Vive, 111 titles compatible with the Oculus (note that some games are compatible with both, so there is cross-over).
- Revive (allowing people to play Oculus only titles on HTC Vive) is rendered non-working by an update to Oculus home, tying the physical connection of the VR headset to its DRM. One day later, Revive works around this, but only for Unreal Engine titles, Unity in the works. SteamVR Oculus-only apparently not affected by this, only those titles purchased from Oculus Home.
- A couple weeks ago, Best Buy sold Oculus Rift headsets in small batches (infuriating pre-order customers who have not yet received their headsets); the Vive is slated to be in retail this June and some have been spotted in the wild, including a Webhallen computer store in Sweden.
- Unity Game Engine 5.4 (currently in beta) will have SteamVR support integrated with the product. Currently Unity and Unreal Engine are popular for Virtual Reality development with several video tutorials becoming available on YouTube. Online courses are becoming available for VR at udemy.com.
- Individuals have been working on experimental VR support for Blender.

Apparently the new medium is gaining traction.