Your argument presupposes that the average test taker will have absolute knowledge of the material, implying that the
bottle-neck to high achievement is critical thinking.
In reality, most Step 1 test-takers have sufficient baseline critical thinking skills (they made it all the way to med school after all) but lack the knowledge. Critical thinking without knowledge is like a gun without bullets. It's just like what
@jonnythan said, most questions, no matter how convoluted and tricky to decipher, will come down to not knowing some fact. Sure, it might be a really tricky 3rd order question with a vaguely described atypical presentation, but even genius-level critical reasoning won't save you if, after determining the disease and treatment, you simply don't know the random side effect it's asking for because you never memorized it.
No one is arguing that critical reasoning isn't important. However, it is only really relevant at the extremes of the bell curve. In other words, if you have really poor critical thinking skills (again, super uncommon for someone who made it all the way to medical school), you will have a very hard time with Step 1, and your IQ may mean the difference between passing and failing. At the other extreme, critical thinking becomes the limiting factor for a tiny fraction of test-takers, those students who essentially memorized everything in UFAP cold. For these people, differences in IQ separate the 260's from the 270's.
...But for the people in between the extremes (i.e. the vast majority of medical students), critical reasoning is not the limiting factor. Indeed, the majority of variation in scores comes down to differences in preparation, namely how much time you took to really memorize the insanely massive amount of testable information.
Extremely few even approach complete mastery of the facts in UFAP. Let's face it. It's a ****load of material. I may have just started M2, but it doesn't take a genius to take one look at the super-condensed, 798 page textbook of "high-yield" facts that is First Aid and conclude "Yep, it's a memorization test." It's common sense, really.