The resistance to flow is not just cross sectional area, it's also affected by the perimeter length, which is the circumference of the pipe's inside diameter. The perimeter length of two pipes that have a combined cross section area of a single pipe is much greater. So the two pipes won't flow as much as a single one.
Look at it another way. There's a boundary layer at the inside surface of the pipe where the flow velocity is much less than that at the center of the pipe. The thickness of this boundary layer is going to be fairly constant, so the area that's unrestricted is smaller for the dual pipes than for the single pipe.
Either of the above could be made up for by making the two small pipes slightly larger than the "equal cross section". Thing is more pipes, even if smaller, weigh more and usually are harder to package. So it's not all about flow.
There's also the deal of how and where do you split that flow, the entry could contribute to flow losses.
good points, and all true, but in most cases , if you do the math , the higher average inertia of the dual exhaust flow configuration aids low and mid rpm power without any significant reduction in peak power,
most cars spend the vast majority of thier runing time well below peak power levels, consider the fact theres very few factory performance cars with a single exhaust, theres a reason!
IF the exhaust system is properly designed and its generally
a bit easier in most but not all cases to fit two smaller diameter exhaust pipes under a lower ground clearance car.
example
look at the chart above
two 3" exhaust pipes show 14.2 square inches of crossectional area
even a single 4" pipe would only be at 12.6 sq inches, and fitting a 4"-4.5" single under most cars and use of a matching muffler will not allow ground clearance on most cars
http://garage.grumpysperformance.com/index.php?threads/calculating-required-exhaust-pipe-size.11552/
if exhaust flow was constant and consistent in volume and speed it would be rather easy to calculate a single exhaust pipe diam and length and cross sectional area to maximize cylinder scavenging.
but exhaust gas is pulsed,(see charts)if you were to think of the mass of a single cylinders exhaust gas as if it was a coke beverage can, dropped through a 2.5" diameter pvc pipe the distance and speed the mass or in this case the coke can travels changes the backpressure or vacuum behind the can in the pipe,the same thing happens with exhaust pulse and the smaller diameter pipe retains the higher pulse speeds even if the pulse is split, remember the goal is a negative pressure scavenging wave arriving at the back of the exhaust valve as it opens.
what came on any factory car, is based mostly on what was cost-effective to produce and easy to install,
and durable over a longer-term, so that they won,t be dealing with recalls or pissed off customers,
parts selected from the factory, and what would pass emission testing is the goal.
very little effort or thought is put into MAXIMIZING PERFORMANCE,
on most cars especially, if its not cost-effective to maximize performance.
if a part won,t last at least a year or more than a basic warranty lasts it is unlikely to be factory installed.
one of the first things you did with that car was add headers and a better ignition..
then they read how that new cam and intake combos add an easy 60 hp in,
HOT ROD OR ROAD AND TRACK...
you'll hear guys say things like " the factory used a 2.5" exhaust pipe so thats all thats required"
but the factory engine was never tested with that new intake manifold,
headers and radical cam you installed nor was that high stall converter or rear gear ever used.
and when you swapped out the 19 lb injectors for that set of 28 lb injectors,
and reprogrammed the CPU..... yeah...
all that stuff was never envisioned under the warranty or considered by the engineers, either.
I can easily remember when those first Z28 camaros came out,
there was a mad rush to swap to decent headers,
as all the magazines assured the new owners it was a fast easy way to add 30 plus hp.
and you were reminded to order the cross ram option if you wanted to be the kink of your neighborhood
http://www.camaros.org/crossram.shtml