For them to have things like:
- high quality wireless video that is decoded on the headset
- decent tracking without anything external (as opposed to using Lighthouse base stations, as used on the Index)
- eye tracking, which enables the foveated streaming (and allows for a significant perceived quality boost)
- on-device compositor with async reprojection (reducing simulation sickness from low frame rate or missed frames)
There needs to be a decent amount of onboard processing, a full OS, and some memory/storage. Before prices exploded (which was certainly after the hardware was mostly/entirely finalized), it would have been only a small price increase (proportional to the rest of the hardware) to bump the RAM and storage a bit to allow for more games to work standalone.
The passthrough cameras are really not a focus of the headset, they are a semi-convenient side effect of the tracking cameras. The actually-decent-quality passthrough is a separate add-on.
If you want an AR screen and only need 3 DoF, would something like the XReal glasses line up better for you?