Looking directly at each other makes silence awkward

This I think is part of why zoom calls are so exhausting, you are not only looking at (and unable to make eye contact with) the person who is talking, but you are also looking face to face with every other person in the call.

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Video calls are not an adequate substitute to in-person communication

As well, it is not natural to be looking directly at everyone taking part in a conversation. Even in a 1:1 conversation often you'll be sitting or walking side-by-side and not looking straight on at the other person the whole time. Looking directly at each other makes silence awkward. There is also an inability to make eye-contact or properly look away (point your attention to some object in your shared environment), which creates an unnatural situation which can leave you feeling much more exhausted than in-person interaction.