As someone who's studied French and Latin (admittedly different backgrounds to Turkish) in the past I would say it's unlikely that we'll get a historical reason for small details like that. However, the fact that there are patterns (bu - bunlar, su - sunlar, o - onlar; ben - beni - bana, sen - seni - sana; etc.) obviously makes it a lot easier to remember!
Since beautiful is single not plural I'm guessing that means the quantity of the subject doesn't need to agree with the form? (Like English as opposed to romance languages like french where a plural subject they/those would pluralize the descriptor to match?). Is this correct?