Cars are shiny to increase areodynamics and fuel efficiency ,and to make the rain fall off .todays paint is waterbased with laquer, when i first started painting cars they still had cellulose paint which was a pig to get a good shine then 2 pack paint was all the rage but very toxic.
Also people like shiny cars due to posing etc and a nice set of wheels makes the car cool.
As for matt paint its pigment seems to be alot stronger for some reason as my ex army landrover was matt green/black. even after going down thousands of miles of greenlanes it never seemd to be all that bad.
Paint on cars these days is very thin i agree all you get is 1 or 2 coats of primer, then your 2 coats of basecoat and then your laquer.
Alot of new cars like prototypes are in matt black and some look very sexy , as for matt cars being produced thats probably along way off,unless you do it yourself.