Would this sound off to a native German speaker? I know that if someone said this in English it would sound very cryptic and odd compared to the "you are better and stronger". When I think of "you are better, stronger", I think of a scientist making some kind of creature, or maybe "the six-million dollar man". It also sounds more poetic.
Same reason for why it is used in English. When you use subsequent adjectives or when you list many things, you use a comma after each one. Typically, you see a conjunction such as "and" or "or," especially in a list with three or more things in it, but there are exceptions.