Because there is a lack of oxygen. Many people breathe, many candles are burning, airing churches is not the easiest thing, especially in old churches.
And the lack of oxygen in the air we breathe makes you dizzy.
I don't remember where, in my opinion, at Huxley's, I read about this. It was about expanding consciousness.
It seems like it's not for nothing that the premises of the temples are decorated with gold, reflected by the light of candles. The atmosphere itself is unusual for a person, plus the smells are harsh (I don't know what exactly they smoke, but I remember that the smell is strong), a high ceiling. All this is conducive to religious revelations.
Well, I suppose, the head is also spinning due to the situation.
The church itself has nothing to do with it. It's all about the setting and the people. Unusual places are often difficult to transfer psychologically, and therefore physically for some people. The same feeling can arise with a new place of study, a new job, a simple trip to the hospital, and other new or rarely visited places. As for the church specifically, such factors as unpleasant at first lighting, crowdedness, specific smell, compliance with the rules of this object, the volume of the room and the number of objects can affect a person. Here, as well as in a museum, gallery or exhibition, eyes can simply scatter in all directions and the desire to be everywhere at once. In general, this is simple stress and nothing more.