It's because in their own unique way, many vaxxed people are actually vaccine skeptics. They don't seem themselves that way (i.e., only idiot, unsophisticated MAGA people would "doubt science") and would never say that they are skeptical, but their behavior says otherwise.
If you are vaccinated and still hole yourself up in your house like a hermit and live in perpetual fear of getting COVID, then it seems as though you doubt that the vaccine will prevent you from getting gravely ill. The people who are most petrified of the virus are people in low-risk categories who are vaccinated with the booster and mask everywhere. It's hilarious.
Remember, liberals had a "I don't trust the Trump vaccine" narrative teed up in the event Trump won in 2020, and if Trump did win you would see much more outright vaccine skepticism on the left, because they are skeptical about a vaccine before it was even developed since they believed Trump was pressuring folks to rush it.