Branding, you're never in doubt when some YouTuber is building a PC using Noctua fans. There's probably some weird psychology in the colour as well. Why would anyone want a fan in such an ugly colour, unless it's really good. The weird beige and brown is highly recognizable, and even if you already don't know about Noctua you're likely to ask questions about the fan solely based on the colour.

The black fans are really only needed by those who build show piece in those cases with glass panels, but they might equally well need white fans, which Noctua doesn't make. Personally I don't really care about the colour of my fans, you can't see them anyway.

Noctua fans have a distinct look, you could say the same about black, if you want to black them out just get a case that hides them since that would look the same.

Why do Ferraris seem to almost always made in red?