As a rider myself I'm sure there's a lot of different opinions but let me answer a few points here.
"motorcycles have to be extra loud so that people in cars take more notice of them"
Is not true. If anything, manufacturers try to make bikes quieter, more fuel efficient and more eco-friendly. For example, Harley Davison, a company know for gas-guzzling, loud motorcycles has been working on an electric bike for quite a few years now with next to no sound, mere inaudible for someone driving a car.
The argument alone about sound is irrelevant as when learning to drive you're taught that bikes are a hazard in your blind spot (see diagram).
I dug around various automotive sites and found this article (meta below), which may not be a scientific study as sorts but covers your question moderately well and gives a general consensus.
Loud pipes save lives
This myth is originated in the very biking world and is has a lot of
supporters among both riders and custom aftermarket exhaust pipes
manufacturers. Basically, those who claim that loud pipes save lives
assume that the louder the noise a motorcycle makes on the road, the
more chances they have of being noticed by other road-colleagues and
thus less likely the occurrence of an accident.
The truth behind such a claim disproves this myth on so many levels:
simple, common physics, common sense or plain reason can bring in
countless reasons because this is just a myth. Let's tread the path of
physics a bit and analyze what's happening from a strictly mechanical
point of view.
Motorcycle exhausts openings are facing towards the rear of the bike,
and it's obviously to the back where the gases and all the noise are
directed. Assuming that the noise a motorcycle makes travels in an
omnidirectional manner is just wrong, because noise is air (or other
gases, for what's worth) in movement. With the air/ gas jest directed
towards the rear of the bike, it's there where all the noise goes. And
if you don't believe this, just 'start your engine in the open and
then check the noise levels when facing the bike and behind it,
When riding at higher speeds, things are even worse, because you're
not only remaining close to the place where all the exhaust gases
start to make noise hitting the mass of air, but you're traveling
further from that zone as you advance on the road. Aftermarket
exhausts can also be almost as silent as stock ones.
A loud vest or
jacket can help others see you better.
High-visibility helmets are good if you want to
be seen on the road.
Now, having established that pipes generate a lot
more noise behind the bike than in front of it, claiming that loud(er)
pipes would help getting you noticed by the man driving the car in
front sounds just silly. A loud exhaust pipe could come in handy when
splitting the lanes at low speed, letting the drivers in front of you
know “something is approaching” and maybe preventing them from cutting
you off or opening the doors. Analyzing the crash reports, statistics
indicate that around 77% of the hazards come in front of the biker,
and only 3% approach from behind. What's next, front-facing exhausts?
Having installed modified pipes on a bike may indeed make it sound a
lot different, with a touch of more aggressive tone, and could, beyond
any shadow of doubt, make the rider feel better about him/herself and
his or her machinery. Cool pipes make any bike look better, there's
too much truth in here to start a debate; but along with the mean
looks come a lot of other things...
Common sense urges us to think about noise pollution: while a rider
might believe that the new sound of the bike is the most beautiful
music in the world, many others might (and will) strongly disagree.
Throttling the bike at 11PM thundering down the alley will simply
increase the prejudice most non-bikers have against us, the riders.
For most people, this sound is as pleasant as a dumpster truck on a
Sunday at 5 AM, after returning from a party and getting one hour of
Adding “it's my bike and I'll do with it as I see fit” is not helping;
au contraire, it just makes a very lame excuse for making excessive
noise. Owning a motorcycle does not come with the right to break the
noise regulations, and even though a racing exhaust might receive
clearance for installation on common bikes, this brings little comfort
to the passers-by and traffic-fellows. It's just a matter of being
polite and care for those around you.