This is why:
These are not specially made, tube-framed shells. These are real cars that started out life in someone's driveway or on a car lot somewhere. You can literally buy a car off craigslist, throw in a roll cage, racing seats and harnesses, a protective underplate, a fuel cell in the trunk, and some wheels and tires, and boom, you have a rally car.
These are not pampered and manicured race tracks. These are real roads that people live and drive on every day. They're closed for the rally, yes, but the rest of the year? They are roads that normal people drive on just going about their daily lives.
There are no rain delays. No bad-weather cancellations. Night, day, paved, gravel, sand, dirt, mud, snow, ice, rain, sleet, hail- it doesn't matter. A rally driver must be able to drive at maximum commitment in all conditions and on all terrain types. A co-driver must be good enough to convey the navigation notes accurately enough so that the driver can trust in the notes even when he can't trust his eyes. The team must be good enough to make repairs and adjustments on the fly and adapt to ever-changing conditions with minimal time or preparation.
These are not lap-based. These are point-to-point races against the clock. The drivers get minimal time to familiarize themselves with the routes. Instead, they must rely on their co-driver to be able to convey the pace notes and navigate for them. It's a bond of trust that just doesn't exist in other forms of motorsports.
Real cars. Real roads. Real fast. THAT is what rally is about.