Not all functions require fixed-width. E.g, I had the page open for a very little bit and it reached a point where the net worth was around $32,674. The function of showing numbers in this comment does not require fixed width. Aesthetics and readability are more important. Both of those are subjective traits. You can optimise for a particular group's aesthetic tastes, or a particular group's readability. Whether or not that requires fixed-width or not is not something I'm able to comment on, however.

Using monospaced when not necessary feels weird, because everything is unnaturally spaced too wide. Some do like the "typewriter" vibe, but in my experience most do not, and that's why fonts have variable spacing.

That's also why most monospaced fonts (and typewriters) tended to make letters such as 'i' or 'l' larger, not only for clarity, but also to fill up the ugly blank space around them. That's also important due to the fact that a blank space has it's own semantical meaning.

I was only talking about numbers, where this issue only applies to the "1" - all other numbers take up more or less the same space.

If you only make numbers monospace they stand out in a big block of text. You break up the ‘texture’, so to speak. Usually the designer doesn’t want that.