The sentinel is not included for a couple of reasons. One is that he may alert others, believing the former king to be an evil spirit. But the more important reason is that only Hamlet can avenge his father's death and he alone must be privy to the shocking truth. The ghost tells Hamlet, who listens, how he died and his sentence in a state of tortured puragatory until the truth is known and the false king deposed: "I am thy father's spirit, / Doomed for a certain term to walk the night / And for the day confined to fast fires / Till the foul crimes done in my days of nature / are burnt and purged away' (Act 3.5.14-17) Then comes the direct order: "Revenge his (meaning the former kings') foul and most unnatural murder." (31-32) Hamlet, stunned, asks for confirmation, "Murder?" and hears shocking truth about his treacherous uncle, "Muder most foul, and in the best it is, But this foul, strange, and unnatural." (33-34).
Why does the Ghost of Hamlet’s father appear but not speak to the officers on sentinel duty?
or, better yet, why is it that in act 1 everyone sees the ghost but, later, when hamlet is with his mother only he can see his father's ghost?
can the ghost "choose" who sees it?