Governments don't tax and license stereo system use.
Your insurance premiums don't go up every time you get caught going over 100 dB.
Unlike a manual transmission Maserati, you can use a stereo with a ruptured Achilles tendon.
Fewer knuckles are scrapped working on stereo systems than on autos.
You can use a stereo system even when the weather outside sucks.
Stereos never have to be towed back to the dealer and repaired while you wait in a room filled with ancient Car and Driver magazines.
You can safely answer your cell phone while using your stereo.
Automobiles wear out from normal use. Stereo systems don't (excepting styli and tape heads).
Stereo gear takes up less space when is it's in storage. But once it's set up, a stereo system takes up more space, but it's cleaner.
Better your 16-year old "tries out" your stereo than he pulls a Ferris Bueller with your Ferrari.