Why did basically every car in the 50s and 60s look like a beautiful work of art...

Differing standards and tastes of the era. Not everyone owned a car back then, you'd typically have one car per household and if you're gonna pay a shitload for a car it damn well better be fancy. Owning a car wasn't so much of a necessity as it is these days so companies had to put more effort into designing it in order to sell. Cars these days are much more of a necessary burden required to achieve anything in life and as a result much of the passion for the hobby has died out as a result. Companies don't have to try at all to make the car look good because you need one anyway and the modern look has shifted towards the "jelly bean" style, anything else would be considered out of date by the average joe.

DON'T YOU DARE TALK SHIT ABOUT MY BABY

Nostalgia for an era you were never a part of. I bet you post YouTube comments on Led Zeppelin videos telling everyone how you're 14 and none of your friends listen to """real music"""

Except for the fact that they brought it to Australia and we've continually redeveloped and improved it to the point where it no longer resembles the original and is a mostly good car, although different eras are hit and miss in terms of looks and reliability.

they didnt
see
youre deluded

Safety regulations. It's mandated that cars be tanks these days. So that's what you get - a boxy heavy tank on wheels. Don't you feel so safe these days.

I suppose at some point they'll mandate the tank treads and there you go, everything will look like a tank.

Well, for those that can't pay to find the loopholes to the rules or just outright bribe the bureaucrats.

And too, ever since the 2000s the economy has been in the dumper and nobody really feels happy anymore. And the inner misery expresses itself in the dumpy uninspired reincarnations of the Chevy Chevette.

Because Americans had enormous amounts of wealth to piss away from the post-war boom and grossly lived beyond their long term means, as visible when everything came crashing down in the '70s.

Because there were no restrictions for car design back then. Designers could go hog-wild making their cars as outlandish and outrageous as possible.
Nowadays they need to worry about safety standards worldwide and regulations for car design.

It'll be different cars, obviously, but the mindset still applies. I also meant mostly pre-2008 because it was a boom time for the economy.
The Impala was a nice looking car back then, but not near the status it now has. The early 2000s equivalent will be aging millennials trying to get a hold of G8 GXPs or VW GTIs and beating off like you are about how things were so much cooler back then because muh rose color glasses.
I don't think you know what the word boxy means. Cars back then are more defined rectangles than anything we have right now. Bloated, yes. Boxy? No.

Lack of wind tunnel testing, no obsession over fuel economy, and design teams that knew they had to differentiate from other brands to get recognition

Also pedestrian impact and crash testing. You can't have sharp edges anymore or kids will get cut in half left and right.