Because people are notoriously terrible at this kind of self diagnosis.

I'm going to answer an anecdote with an anecdote here, but I was getting headaches that were completely correlated with food containing MSG. I "researched it" (cursorily google searched) and found that MSG was totally safe so I didn't want to believe it, but the correlation was there. Later I found out I had high blood pressure—the high MSG foods were also high sodium and the sodium was kicking my blood pressure up really high and causing headaches. When I got my blood pressure under control I tried MSG again and no headaches at all.

The body is complicated and things aren't always what they seem to be to our pattern seeking brains.

Yes, for something like “msg causes headaches”, that needs to be blinded to discern if there’s a placebo effect happening. It’s not nearly as straightforward as “touch hot surface, get burned”.

Parent brought up A/B, the person you're responding to asked if it was blinded. Doesn't seem like the affront you're making it out to be, it's an innocuous follow-up question.