For the same reason a lovely piece of fish or steak is delish when broiled - but disgusting when boiled. It has to do with what happens to protein strands when different forms of heat are applied to them.

That's why cheaper cuts of meat are tough as hell and virtually inedible when broiled/dry roasted, but delicious when braised for a long time in a liquid. Imagine boiling a crown roast or prime rib. Could you eat it? Well, it won't kill you if you gag it down. But why would anyone want to? Ditto for eating a 2 hour boiled egg. Or a boiled, rather than roasted bone/marrow.

How protein is cooked is crucial. That's why, btw, the chicken you use to make the stock is also 'inedible' and is usually tossed with the spent veggies. Since the chicken has been boiled rather than braised it's tough and tasteless. Could you eat it? Sure, but again - why would you want to?

This is why stock scum is inedible, and why it's skimmed. It has nothing to do with 'cloudy' stock, since skimming the scum off does NOT make a stock clear. Only clarifying the stock after it's done and the spent chicken, veggies and spices are removed makes a stock clear. There are several stages and methods for this, depending on the final product you desire. If it's demi-glaze, no need to do anything. Ditto for a stock to be made into a rustic soup at home, for instance, or used to make a sauce.

But if you want the stock for consomme or aspic you'll need to do several stages of clarification -- such as pouring the stock at least twice through many layers of cheesecloth, after which you reheat the stock, bring to a simmer, and add 2 egg whites.

As the whites cook and coagulate, they'll whisk up whatever impurities remain and float with them to the top of the pot, where they can be skimmed off. Could you eat the resulting protein mass instead? Yes . . . but I can't imagine wanting to do so.

Lisa