Because the writer wants to create a kind and strong maiden who is waiting for Shinichi.
After all, the reason the writer has given is baseless and fans who are supporting it are just pawns trying to prove a baseless reason right created by the writer that if he tells her his identity then she will be in danger
According to me, only foolish people will support this. It doesn't matter if Shinichi tells her or not, her life will be in danger because the organisation will kill her too upon knowing Conan's identity no matter if she is innocent.
And according to me, Rachel (Ran) would have supported him like Hattori. After all, if she had known about the organisation, she can also take necessary precautions and can cover his identity more effectively.