For a number of reasons. There are drugs and other substances in tea, one obviously is caffeine but I believe there are others whose effects may not be known well or known at all. The effects of tea are noticeably different from coffee. It's not just stimulating.

From the English writing that I referenced. To me it sounded like there was something there, particularly given my personal experience. Dullness, I believe they may have used the term "dullard" to refer to a habitual tea drinker.

The fact that billions of people daily take tea that means a lot of economic interests are at stake. This can cause propaganda, some very obviously dumb, but some in a very sophisticated way to maintain those economic interests. The Chinese study I referenced is an example of dumb propaganda. There's no way that drinking tea can prevent or cure Covid-19. Did they think people would believe that? The NIH has a disclaimer on this study but still why they have it in their library is a mystery.

I am in no way saying that I believe tea has negative effects. I'm just seeing if anyone has any personal accounts of them.