Reducing sauces

When I follow a recipe for a sauce it often talks about reducing by half (or whatever) to "thicken". But that *never* happens to me - it simply doesn't change in consistency at all until I have about only a tablespoon left in the pan.

For a while I'd just presumed it was me lazily using supermarket liquid stock (e.g.

but I recently made my own chicken stock, and that is pretty much the same (made from a roast chicken carcass).

Is there just a missing assumption here that the stock to use is heavy in gelatin? Or what?

Why don't supermarket stocks like the above thicken? Same goes for their beef.

I can always use a cornstarch slurry, but I'd like to understand why my experience doesn't match up with any of these recipes...

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