A lot of alternative visual designs run into problems with the technical realities of modern UI tooling. I'm going to talk about web tech, but Android layout works largely the same, last I checked (one trillion years ago, to be fair).
The futuristic UI look that Arwes is doing comes with a whole lot of animation, which is very cumbersome on the web. The CSS transition property is good, but it works on a very narrow range of properties, and anything involving inserting/deleting/adding elements needs a whole song and dance to trigger the animation then go clean it up when it's done. You can do it, but it's a whole lot of extra work.
Most other stylized looks involve big chonky stylized borders and background graphics. But we generally want the same style to work on mobile, and the chonkier your borders, the harder it is to cram it into reduced space. (Also CSS still doesn't have native 9-patches for some absurd reason. It used to be how everything was done, until the world agreed to stop abusing tables.)
Meanwhile the modern flat look is easy to implement in CSS and doesn't have any trouble scaling down to mobile. And it's still a good enough look, it'd even be kind of striking if it wasn't ubiquitous.