Because those in REAL motorsports, I mean, PRO motorsports, can get the most out of the engine with a manual transmission, and hence, KNOW HOW to do so.
Unless you are REALLY GOOD with a shifter in hand, us "civilians" fare way better with an automaticl. It's the time between shifts that kills you, and there are tons of programmers out there to make your autie-matic a helluva lot more refined with a better shift pattern, and at the right RPM's as well. I know very few "civilians" that can truly handle a manual like the experts do.
I have a truck that I use to haul off garbage, and with the proper maintenance, has reached 325,000 miles WITHOUT ANYTHING being doen to it major, other than the usual stuff that goes quickly, such as tires and brakes. The clutch and pressure plate, not to mention the throw out bearing, are ALL original.
That used to be my daily driver, way back a few years ago. I am, quite simply, tired of shifting. Hence, my automatics.... ;)