Why does the poet say "all the men and women merely players"?

It's one of the most famous lines in literature:

All the world's a stage,And all the men and women merely players . . .

This famous saying comes from Shakespeare's play As You Like It, and it's spoken by Jacques. What he's saying here is that simply going about our daily lives is a type of performance. Just like actors on a stage, we all have roles to perform: Daughter. Student. Sister. Captain of the soccer team.
The fact that Shakespeare writes that men and women are merely players suggests that the roles are preordained and that a person's choice in being a mother, for example, or a doctor or starting point guard for the Brooklyn Nets is out of their hands. What roles a person performs over the course of their lifetime is up to fate.
The notion that "all the world's a stage" has entered pop culture, but within the context of the play, this idea does not sit well with Jacques. He's a pessimist, and the idea that everything is up to fate makes him feel like life is predictable and pointless.