probably because
a) movie stars etc are some of the most photographed and video'd people, therefore it is easier to find a fitting expression/situation than to try and capture one yourself
b) a recognisable face is more relatable
c) not many people want to put their own picture online (where it can easily be misused and abused) and even shallow people with 30,000 selfies are still just pics of them "trying" to look glamourous rather than expressing a range and variety of emotions
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