i think they don't know who she is because she is such a klutz in real life they think "There is no way this silly girl could be a super powerful guardian of good blah blah blah"
Real reason: plot, the bad guys cant figure there identities out because the writers will it, just as super man is never descovered to be clark cent despite superman not wearing a mask and every time supermans some were clark cent is no were to be seen
It would all end to soon. Like in a crimi. They cant find out everything in the first 20 minutes. But some guesses of the villains on who is Sailor moon very totaly stupid. Like when they mistook Naru for Sailor moon. But there were some who figured it out. The police friend of Mamoru found it out.