It seems every time ST folks are in civvies, each one is in a wildly different style
Perhaps this was intentional? After all, the Federation is supposed to be made up of many different worlds. And while "western" clothing in our world has a general sameness, there are still regional differences. I'm not a fashion expert, so I cannot quantify the exact differences. Yet many times I've seen, say other students at an international university, that looked somehow "different." I later learned the regions of these perceived differences.
"Look at the cut of those trousers! The guy must be a Capellan. No doubt about it."
While the Federation may have routine commerce between planets and maybe even subspace Internet sites, the Denebians know that the Vulcans make the best fuel synthesizers, and salt vampires know how confusing and boring it would be if everyone looked the same.