Why can't there be speaker systems in electric cars to give the driver the V-8 experience?

The GR86/BRZ added those in 2022+ but didn’t offer a way to control loudness, using a small board attached to both the canbus and the audio system. It is, in online enthusiast communities, disliked for providing ‘fake’ horsepower and for being ‘too’ loud. So: they can, but if the manufacturer gets it wrong, then it’s much worse.

This solution also misses that for many folks in the U.S., imposing the emissions of a V-8 engine upon their environment is not just about how it sounds in-cabin, it’s about marking territory, and V-8 engines are quite good at that. You would need a subwoofer to broadcast it properly and you would likely run into amplified sound laws in many regions that are not applicable to engine noise.

Some cars do that (and let you choose between V8 vs spaceship sounds). Even though it's optional a lot of people mocked the feature

They can and do. Since at least the 10s automakers have been using tricks like pumping the engine sound into the main cabin or using speakers playing an audio file. The speakers usually toggle when the car is in "sports mode" or not. Car enthusiasts typically don't like it because they know they're being lied to. Average drivers don't even notice it because we all grew up with "car goes fast, cat gets loud." Automakers started doing this to make inline 4s sound growlier and also because most of the time you want your car to be as silent as possible. Obviously as EVs appeared around the same time, they did the same thing with EVs (eg the BMW i8 is a notable example).