Funny you should ask that, I was just trawling around looking for memes to post on !gothindustrial@lemmy.world and came across this classic image:
It's from like 2007 but still got a lot of truth. See here for a breakdown of the various "stereotypes" of goth types by the same artist, many of which do not involve monochrome: https://www.deviantart.com/trellia/gallery/25271332/goth-stereo-types
If you check out pics from last summer's Wave Gotik Treffen, you'll notice that yeah, black is the baseline but it's not exclusive: https://piefed.social/post/883617
As for your question "why?" idk but wikipedia says:
The anchor that holds all elements of dark culture together can be viewed as the color black with all its associated symbolism. It is seen as an expression of seriousness, darkness and mysticism, but also of hopelessness and emptiness, melancholy, as well as its association with mourning and death.[2][6]
https://en.wikipedia.org/wiki/Dark_culture
But a lot of tradgoths will say you don't have to dress or act any certain way to be "goth", you just gotta enjoy the music. idk tho, I'm more into industrial!