"Why is mainstream America so bold in our other aesthetic choices and so meek in our automotive ones?"
Because cars are a durable good, designed to be held for multiple years and then sold to another buyer for a substantial sum of money.
Because getting 50 strangers together with rattlecans will give you a paint job that looks good for about six weeks and then it will fade and be absolute shit.
Because most people want their cars to be an extension of them - they do not want to be extensions of their cars.
I say this having built a 4x4 Triumph TR-7 in primer gray - I've driven memorable cars. I had a buick with only three doors. I've made city council meetings with my vehicular choices. And here's the thing: The general reaction of everyone on the road to everyone else on the road is dislike and disdain. When you become noteworthy in that miasma of uniform disdain you become a lightning rod. Yeah, traffic was bad, but that asshole in the graffiti Lancer....
And I mean, look. Here's what it takes to paint a car. Porsche is being special about it because of course they are (colors outside of their selected range are $3k extra) but it Kias aren't painted significantly differently. It's an ordeal to make a car pretty for the life of the vehicle and it's a stone cold pain in the ass to do it custom.
You wanna see what customization looks like?
That's a truck. That guy lives in that thing. Drives it all day every day and that's how he signals his truckdriverness to all the other truck drivers. The rest of us? The rest of us don't define ourselves by our cars and we don't want anyone else to, either.
So. black, red, white, silver.
It put me in mind of an early review of the Ford Focus RS. The review expounded upon the car’s nimble handling, its sprightly pickup, how fun it was to drive. In the comments, however, dozens of commenters said that the car sounded great, but that it looked too memorable for them to be interested.