Cyclists usually do know a city pretty well. I know I do. I was a delivery driver in Austin, though, so that might have something to do w/ it. I never get asked directions though. But, I think people assume that cyclists know where they are going b/c of several reasons:

-If you are on a bike, it is likely that you live in that city and are not visiting (especially if it's obviously a nice bike and not some rental cruiser bike)

-If you ride a bike everywhere, you probably are familiar with different streets and how to get places

-Being on a bike makes you look like you know what you're doing (usually).