Why can "low" become lower (comparative) and lowest (superlative), while "up" can become only comparative (upper), rather than superlative (uppest)?
The second question is what does act as a substitute for superlative of "up"? I believe that it's needed in the language.
Editing:
After reading some answers here who claim that the word "up" is not an adjective and "upper" is not the opposite of "lower". I had to support my initial premise by Cambridge dictionary that shows that there is an adjective which is called "up". In addition in the same dictionary the word "upper" is marked as adjective and the word "lower" is marked there as a opposite. unlike the most of the answers here.
In addition, what's about "more up" is the following context "If you feel a bit depressed today, maybe your mood will tomorrow be more up." Is this not considered as a comparative adjective of "up"?
Best Answer
Well, the opposite of low isn't up but high, which has the comparative higher and the superlative highest. So they're equivalent in that regard.
The opposite of up would be down. But up and down when used to describe relative directions are adverbs, not adjectives, and they don't have direct comparative and superlative forms (unlike say, badly, worse, and worst). While I don't see anything technically wrong with saying more/most up/down, that's not something you really hear in practice. Instead we can say something is farther up/down or farthest up/down.
Upper is a plain adjective, not a comparative. You can say: John is in upper management, but you wouldn't say John is upper than Jim. Also, while downer is a word, it's a slang noun and not an adjective. downest is generally not recognized as a word as far as I know, but neither is uppest.