Rosalind keeps her disguise even after reaching the forest safely. Why?
Rosalind does not necessarily assume that she is safe just because she has reached the Forest of Arden. After all, Orlando did not consider it a safe haven when he assaulted Duke Senior and his companions and demanded food. When Duke Senior invites him to join them, Orlando replies:
Speak you so gently? Pardon me, I pray you.
I thought that all things had been savage here,
And therefore put I on the countenance
Of stern commandment.
After she encounters Orlando in her disguise as Ganymede, Rosalind must find it amusing to let him continue to think she is a young man. One of her characteristics is that she has a sense of humor. And at the same time she can continue to act as an independent male in dealing with the locals. It is some time before she encouners her father. In the final scene she is able to reveal her true identity. Celia seems like a very passive character throughout, content to play Rosalind's game as long as Rosalind wants to maintain her disguise.