It's even simpler than that at this point. Ignorance was a valid excuse a year ago, but with all the data we have, you'd have to actively delude yourself to not know the truth.

And that's the key: active delusion. Because these people already got the shot. They believe in it because they have to in order to maintain their own sanity. To admit that it's placebo at best after you've got it means admitting that you were duped. People are more concerned with losing face than anything. They don't want to look at feel stupid, so they continue to play along.