Because it goes both ways. For both kitchen sinks and themed worlds.But that's also true of campaigns with no restrictions on playable races whatsoever. So it follows, then, that your theme/genre/tone tangent is completely orthogonal to the question I asked.
Many of the argument over race (and class) are really about theme/genre/tone/style. But players and DMs frame it around race and class.
My guess is because race and class are one of the few things mentioned in both the PHB and DMG. Themes, genres, tones,and playstyle is often only in the DMG and it is easy for a DMG to ignore it or forget it.
And without a shared language, people talk over each other.