I didn't get through all the comments, but was surprised to see no one elaborate on the mirror. Your reflection is mirrored compared to the photo. Faces are not symmetrical.
Most people predominantly only view themselves via reflection, so when they do see themselves straight on, it doesn't match up with what your brain "knows," and hence there's an inclination to treat it as "other," not "self."
There's probably more than just that factor at play, but visual perception is something humans rely on, so your brain is on point for recognizing the difference between the two views.