I never succeed in thickening sauces with pasta water. What am I doing wrong?
You seem to have the wrong expectations. No, it will never be as thickening as a cornstarch slurry. If that's the level of thickening you expect, you are really better off using the slurry.
Don't forget that pasta water thickening is a traditional technique from the time when people did not go to the supermarket to buy a pack of cornstarch. They cooked down ripe tomatoes for several hours, and the starchy water saved from needing a few more hours of evaporation. Also, they cooked with homemade pasta, which had some flour residue sticking to it, not the perfectly-gelatinized industrial pastas we buy today.
If this is not how you cook, and if you prefer pudding-thick sauces, then the slurry is probably the better method for you.
I notice Kenji from Serious Eats has also tested pasta water and recommends it for flavor reasons. He also tested it for thickening - but against salted water, not against a slurry. That's what people mean by &quot;it thickens&quot; - it thickens when compared to random liquids, not when compared to thickeners.

There is also another thing that might have been meant in connection with &quot;pasta water for thickening sauce&quot;. It is actually a very specific kind of sauce: an emulsion between pasta water and a liquid fat. There, you start with the pasta water, cook it down sufficiently, and stir in the fat into the hot pasta water. It produces a &quot;thick&quot; sauce - not as thick as, say, mayonnaise, but it is certainly thicker than the oil you are putting in, or than the pasta water itself. It may have been in this context that you have been hearing of &quot;pasta-water-thickened sauce&quot; and thought it meant that pasta water is a thickener for random styles of sauce.