Not to insult US drivers, or imply that UK driving standards are fantastic, but I believe the reason you can't simply trade in your US license for a UK one, so to speak, is that some US states have woefully inadequate testing standards.
The UK driving test is pretty tricky to pass compared to most countries, and I guess, since the standard in some US states isn't up to scratch, then everyone from the US suffers. I suppose they could set up reciprocal agreements with states which cut the mustard (I believe Germany has such agreements with selected states) but I guess that would be too much trouble for the DVLA to bother themselves with.