I would like to know why 'Who is there?' is correct and why we can't use 'Who are there?' even if we know there is more than one person.
Who is used as a singular or a plural pronoun. If you use the who as an interrogative pronoun for a person, it takes a singular verb. If you use it for more than a person, it generally takes a singular verb, though the use of a plural verb is not ungrammatical. So both the following sentences are correct.
Who is there.
Who are there.