Semantically, you is both singular and plural, though syntactically it is always plural: it always takes a verb form that originally marked the word as plural, (i.e. you are, in common with we are and they are).

Why is you considered plural?

It is only the singular pronoun 'you' that takes a plural form. It just so happens that the first person singular verb for for all verbs except BE has the same form as the plural. (You was originally the second person plural form. It has now replaced the old singular form thou in standard English.)

Can we use you for plural noun?

English has always had a gender-neutral second-person plural pronoun: you. You has been the second-person plural pronoun since the days of Old English. You has always been everyone's second-person plural, from Beowulf to both the Queens Elizabeth (or if you prefer, both the Queen Elizabeths).