Because when they do, they start charging more for them. cough Kia/Hyundai

Lots and lots of reasons.

I’m basing this on your comparison of normal cars to currently existing exotics.

Predominantly: The vast majority of people don’t want an exotic car. They want to go from home to work and the store, maybe a drive for a leisure trip. They’re boring. They want to get their stuff and people in and out of the car easily and conveniently.

Exotics do not do convenience well. There’s minimal trunk space, there’s space for only two people, often “snugly.” They require some contortions to get into and out of. Think of how out of shape many people are and see if they fit into a highly contoured, reclined, and snug race seat and can crawl in behind a scissor- or butterfly-style door with a very low roof.

Engineering-wise exotics are expensive, both for the manufacturer and customer. Those compact, low, aerodynamic bodies on exotic cars take a lot of work to pack all the mechanicals in along with having to design a body that is crash-worthy for each new style. On top of that, they’re often mid-engine, which means a lot of specialty parts like transaxles, and wildly different handling characteristics than the average consumer is used to when you shift weight to the back of the vehicle.

Manufacturers stick with the “boring” designs because they’re based on existing engineering that is safe, requires minimal cost to make the new iteration, aerodynamic, fuel efficient, and has proven to be sellable to consumers. Profit is king. They’re not going to take chances on crazy styles that may not sell because again, people are boring.

I know people are going to chime in about mundane cars in production today that have some of the features I mentioned and treat them as an exception that invalidates the opinions I’ve offered, but the point is that if they were economical and profitable designs in an exotic body they would be more widespread. “You could just take “x” engine and transmission and build a “y” around it” argument.

I would suggest maintenance is a potential cost problem, too…some exotics literally require the car be split - the entire rear of the car containing the engine and transaxle removed from the rest of the car for access because of the compact engineering and inaccessibility to some wear parts. However if Toyota made a low-buck supercar looking commuter car I’d hope the maintenance would be cheaper and easier.

So there you have it. Cost of design, engineering, and maintenance. Boring consumers, convenience, and safety. Affordability and profit. That’s why we don’t have exotics everywhere. The market has determined that the few Halo cars we see like the Supra or C8 Corvette, or even the Mustang, is all the market will bear.

I would argue that it is already the case that cheap cars look and perform excellently, compared with cars produced fifty years ago. They are more reliable, economical, comfortable, higher performance, superior in virtually every respect.

The other factor to consider is the use case. Something like a Ferrari is not reliable compared to a VW Golf, it sucks at carrying passengers and cargo, terrible fuel economy, it is horrible value for money and inferior in most ways apart from one - compensating for a small penis. That is its chief purpose and it is supremely well crafted for this use case.

Source: automotive engineer of 25 years.

They have “kit cars,” which are all the parts you need to build a fancy vehicle on the chassis and drive train of a normal car. When I was young, Ford Pintos were common chassis for kit cars.

They want to “reserve” features looks and styles for expensive cars to make you upgrade.

Some companies spend a lot of money on market research and trying to get ahead of trends (Mercedes Benz, Honda, Toyota) some companies disregard common sense and do what they want (Alfa, The French in general) and cheaper brands dont waste the money on price point cars. Its not an Iron clad rule but people buy a german to project wealth, you buy other euros to project style, you buy Honda and Toyota for reliability. You buy a Nissan Altima because they will finance you, you buy a Chinese car because you arent keeping it past warranty expiry anyway.

As to what sets them apart. Little things like painted brake calipers, the quality of the badging, the texture on the plastic interior, little trim pieces that stop you seeing any of the interior workings, the windscreen wipers looking “chunky”. Wheels and stance also play a large part of the image. Wide wheels simply look more expensive, as do lower profile tyres.

Then things get a little more tactile, the dull thump when you shut the door over the higher pitched clank, the thickness of the interior plastics and number of fasteners making the interior feel sturdier even though you cant see the difference, the sensation of the indicators being put on, the UI on the touchscreen…

Prestige brands also dont do trim level names/badges very often. They like letters and numbers like 330i M-Sport or c65 AMG. Lexus followed suit with the LS400. They WANT you to say “Yeah, I got the Touring package” or “I bought the AMG sports pack” and they know their owners want to do it too.

I would like to mention manufacturing. Mass production of body panels. Those sleek looks may come at a cost.

Another thing is kit cars, basically get a chassis and replace body panels. Lawn mower engine…

https://en.m.wikipedia.org/wiki/Kit_car

I think our 2021 Honda Civic Sport looks pretty fancy. No one is going to mistake it for a Ferrari, but I think it’s pretty hot for the price.

I also wouldn’t fit in any real sports car anyway.

Cheaper cars have been taking design cues from supercars for at least a couple decades

it’s subjective. Unless there’s obvious rust or off-color replacement parts, they all look the same to me.

why don’t manufacturers of basic cars just put a fancy-looking exterior onto them?

They do. This is exactly what Hyundai does for the Genesis brand.

I didn’t know I wanted to slap a Lamborghini-style chassis onto a lawnmower, but now I do.

but why don’t manufacturers of basic cars just put a fancy-looking exterior onto them?

“Kit cars” are a thing…

Not sure what’s popular these days, but for a while people were putting Shelby Cobra bodies on Miatas.

It’s way more than a Miata, but way less than an authentic Shelby.

So people who just care about the looks have been doing this for decades now.

But when it happens as a production, people don’t buy it because other people recognize it for what it is, look at the PT Cruiser.

So if a couple people do it, it passes as expensive. If a lot do it, it comes off as tacky and becomes a joke.