Interesting question. To clarify, in English "you" is used for the second person singular and plural, in both polite and informal/casual/even rude contexts. Basically, English has much less formal structure in general.
In Spanish, my hunch is that Usted being a third person is because often you are referring to that person as Usted because speaking indirectly is more formal. You would be using this person's title (Dra. Ramos, Sr. Rivero, Srta. Diaz, etc), not their first name, so you in that formal, indirect space both communicatively and grammatically. Think about, even in English, in older films and plays, if a servant is speaking to their boss, they speak in the third person many times, such as "would the doctor like a cup of tea?", even though they are speaking to the doctor. It seems that when the situation calls for more deference, speaking directly seems too blunt or rude, too assuming of closeness for the relationship.
Does that help?