That's not true, there are cars that can break noise regulations stock, even mass production models: https://www.thedrive.com/news/stock-hyundai-elantra-n-fails-...

The key difference is even loud factory exhaust setups leave room for relatively quiet operation depending on how you drive.

That Elantra driver was an idiot: he went in a town and drove in sport mode and drove in a way that forces extra loud pops

If he hadn't been forcing it the car was perfectly capable of not making excessive noise.

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The $100 bozo special on the other hand has none of that: it's always loud and obnoxious.

In fact it gets more obnoxious at cruising RPM because of drone.

There are aftermarket exhausts that try to match the factory-style "not loud by default", but they're expensive because it takes careful engineering and integrating things like valves and Helmholtz resonators.