Because the subjective experience is the thing we actually care about.
Same reason you ask the users of any product for feedback. Sure, you can objectively see that they were able to click the register button, still doesn’t guarantee they came out of that experience wanting to use the product.
are you under the impression that the sole focus of an independent review as described in the root comment would be to investigate the personal veracity of "Participant 1"'s narration? do you alter course in your product because of single, particular user anecdote? i'm not sure what you think you are arguing against here...
Same reason you ask the users of any product for feedback. Sure, you can objectively see that they were able to click the register button, still doesn’t guarantee they came out of that experience wanting to use the product.