The word "you" used to be plural, now it's singular. Now, "they" can be singular or plural. It's hard for a new pronoun to come into existence, it's easier to repurpose what's already there.
There's the pronoun "one", but that seems often used for indirectness. "One might put gas in the car before one leaves town" is a little more indirect than "Put some gas in the car so we don't run out of gas in the interstate like the last time!"