Why does this look like it’s from the 80’s still?
The underlying architecture of NeXT was largely adopted into OS X and iOS.
The GUI appearance evokes nostalgia for those that used it (the very first web browser was written within NeXT).
FWIW GNUstep is themable so it should be possible to customize it. Though i'm not aware of any that follows modern trends, but at least it is possible to make it kinda sorta like a broken telephone version of early Mac OS X using the Rik theme[0] (the screenshot also seems to be using a custom theme and setup for Window Maker too and judging from the shadows it most likely also uses Compiz as a compositor).
(Its a big reason why I stopped contributing to GNUStep, as the resistance to modernize at least a little bit was large).