A few reasons sprung to my mind-

• The splines of standard cassette cogs are not big enough and not designed for the forces fixed gear riding would exert on them.
1) Also, cassette cogs are tapered to aid in shifting, fixed cogs are a different shape since you do not want the chain to easily slide off of the teeth.
2) If not using standard cassette cogs you'd need a new standard which = proprietary (which = expensive)

• Weight. Adding bunch of extra metal to the hub to make the chain line adjustable would add a significant amount of weight to the hub.

• Since chain line can already be adjusted at the BB using spindle length there is not much market for a hub with adjustable chain line.
1) Why buy an expensive custom hub and have it built into a wheel when a new $25 BB could solve 95% of chain line issues?
2) Would the hubs be 120mm spaced, 130mm spaced, 135mm spaced, or would you provide all 3?

• As for "stripped cogs" and "stripped lockrings" these are usually the result of improper installation and not a flaw in the design of that system. Lots of people have been riding traditional cog/lockring setups for a long time and most of us have never had these issues.

• If you can't or just don't want to worry about threaded cogs and lock rings, Miche makes a thread on 'carrier' for their proprietary cogs (somewhat similar to what you describe but it still uses threads and is designed to simplify changing gearing at the track) or you can get Tommy cogs which use the standard 6 bolt brake disc mounts to screw to the hub.