Measuring quality directly would require watching and in detail analyzing the specific actions of people providing treatment. And the watchers and analysts would have to be experts, and would have to watch a non-insiginficant portion of all medical care provision (and not just in this country but also in other countries if your trying to do a comparison). So you would need armies of experts, a rather impractical scheme.

You could try doing the same with a tiny portion of medical treatment. A hundredth of a percent would be affordable. But it still wouldn't be cheap, no one wants to spend the money, and also a smaller portion would give less reliable results.