Should motorcycles have louder sounding exhausts than Cars? And why?

No as they cause a nuisance.
They do not act as a safety device when loud as the noise heads backward and not toward a driver that might pull out on a biker.

I read an article years ago about a study of safety at train crossings. Something like 2 dozen people are killed in the US every day crossing train tracks. The train makes plenty of noise but they don't hear it! Because the sound streams out mostly to the sides and back, but where you really need it is out front.

Aftermarket exhaust systems are one of the biggest-selling accessories, for all kinds of bikes. Bikers insist they add horsepower, but all they really do is make more NOISE. It's not about more safety, it's about getting attention. And it's REALLY ANNOYING when one of these cretins comes past your house at 2:00 AM.

No, we do not need anymore noise pollution than is already there.

Car exhausts are a) longer with baffles of around twice the volume of motorcycles and b) surrounded by sound-deadening metal. Physics does the rest.

I'll go ya one better: motorcycles should be louder than a B52 bomber at full power on takeoff.