Trendelenburg actually does work. It increases mean arterial pressure, cardiac output, cerebral perfusion pressure, and coronary perfusion pressure, and it does so without diluting hemoglobin or clotting factors.
Yes the effects are transient (the effects of passive leg raise are similar and last longer), but in profound hypotension it can be potentially useful as a bridge to other therapies.
The studies that show "it doesn't improve outcomes" were probably done on trauma patients who presumably were going to keep bleeding - and die or not - regardless of almost anything that was done in the field. It may be worth pointing out that prehospital IV fluids rarely improve outcomes either; all things being equal if you had to choose between one or the other, I don't think it can be said in most cases that IV fluids would result in better outcomes than t-burg.