Because you want to compete in a different market segment.

To put this very concretely, look at Pimax. Their Crystal headset allegedly has standalone capability. All the hardware is there for it, they make this thing, they sell it -- but they don't have a standalone platform, and have no hope whatsoever of being able to develop one.

For PCVR, they just use regular standard SteamVR and OpenXR, but for standalone usage, they suddenly have to be an amazing software company and they definitely are not. Using Horizon makes a ton of sense, and makes their product more attractive and better than it is without it.