Why noun you is treated as plural?

Answer 1

You was originally plural or formal. The singular pronoun for someone you knew well was some form of "thou".

That said, you isn't really "treated as plural". It does take the same verb forms as plural nouns, probably at least partly because of its history.

Answer 2

In English, "you" is both singular and plural, so it can be used to address one person or a group of people. This can cause confusion since the verb form is the same for both singular and plural "you." It is a unique feature of the English language.