This isn't some huge statement on how millennials want to be grownup and childish at the same time, or some other pop-psychology drivel. It's just the normal change of fashion over time. Non-flake paint jobs stand out a lot in a sea of cars with them.
In fact, I've already noticed a shift back towards paints with contrasting metal flake now that even the most pedestrian Japanese cars are coming with simple glossy paint. BMW has this new sparkling copper grey that's a cool grey in low light, a warm light grey in direct light, and has reddish specular highlights and it has this very 90s color changing vibe to it.
> UPDATE: A few ppl have also mentioned the important role of the “Nardo Grey” paint color from (Porsche-owned) Audi in this timeline — a wet-putty hue that came out in 2013 and became a car-head phenomenon.
Grabber Blue came out on the Mustang even earlier and is a throwback to the 70s color of the same name. Voodoo Blue and Army Green on the FJ Cruiser is also a simple glossy color on an even older car. I'd argue this kind of paint was popularized by the retro-modernism of the 00s mixed with the cool grey trend of the 10s.