Because the average roadie doesn't build up the leg strength that a fixie rider obtains by only having one gear. They shift too early when the going gets tough and they don't know how to work each gear just enough before downshifting.

But watch out, the very above average roadie will smoke you on a decent hill. Of course, alot of very above average roadies train on fixies.

I know my climbing strength (geared or not) jumped up a level or three after riding around with one gear for awhile. After having only one gear you learn how to get the most out of each gear on a shiftie.