Ever wonder why we say "I am" and "he/she/it is" but "we/you/they are?
In older English, the pronouns were declined as follows:
singular first person: I (am)
singular second person: thou (art)
singular third person: he, she, it (is)
plural first person: we (are)
plural second person: you (are)
plural third person: they (are)
For some reason, English dropped the "thou" and extended the use of "you" to include the singular as well as the plural of the second person.
Since "you" is plural, it kept the plural form of the verb "to be." Thus we have ended up with the following conjugation of "to be:"