Probably because I was always hella aware that something really bad would happen if I slipped up so I always assumed other people were also aware of the difference between their own private and public lives.
Which I think is the case for many people. But also, many especially want to believe their public life is the only one true personality and now it's a whole other problem. Because what makes you popular is normally what makes you dysfunctional. And when you insist on believing the false persona only, you deny other parts that make you human.
It's not a coincidence that the most socially successful person you know is also the most psychologically challenged.