Is because they are the vest they know what to do
The next soliloquy Hamlet has after seeing the ghost of his father is in Act II, Scene ii after the players, Rosencrantz and Guildenstern, have left him alone. In this soliloquy ("what a rogue and peasant slave am I"), Hamlet expresses his frustration with the fact that the actor could create tears in an instant about a fictional character, but he has lost his actual father and cannot even do anything about it. Through this he also decides on the plan to try and catch Claudius' guilt.
