I think we should also not forget the importance of movement in how we see ourselves. Smiling is a process. It has a beginning, a middle, and an end. In the mirror, you can see yourself making all the phases of your smile. In a photo, you only see one frame of it, and so the picture is flat and emotionless, even though you're "smiling".
How many pics have we all deleted, because most of us looked ridiculous in the single frame, even though we knew that particular moment was full of hilarity or joy? Meanwhile, the movie of that moment would have shown us all in a glorious light, living our best life.
We are not meant to have our lives captured in single frames, to be printed and placed in a book.
Another way to think about this is to pause the TV at random points. Even those unreasonably beautiful people often look weird when you take a single frame that hasn't been carefully selected.
This doesn't happen in magazine photos because a professional is very carefully selecting angles and poses and lighting such that the still image doesn't look weird. But most normal people don't know how to do that well so they pretty much always look worse in photos than in real life.
Still images just don't naturally capture how a human looks since our minds are used to seeing movement and kind of averaging how someone looks, not picking a single moment and considering that representative.
Add in the focal length issues other commenters have talked about and basically everyone looks worse in photos than real life.