Actually, you is the plural form. The singular is the ultra-dorky sounding thou/thine/thee. Thou is the subject (anywhere thou wouldst use I), thine is the possesive (it’s like mine), and thee is for every other use (anywhere thou wouldst use me). Oh, yeah, using thou in the most proper manner causes the verb to do some funny things. See above.
So why you for one and all? I’m not sure why. I’ll let a real academic answer that. All I know is that historically you crept in on the territory of thou so much that anyone that uses thou sounds kinda dumb or is reading something old.
