I bike about 5000 miles a year in the city and have never needed to carry my bike down stairs while riding. Once you get it home, you might need to lug it up to your apartment, but it's well within the realm of possibilities for a healthy adult to carry a 40 pound bike up a few flights of stairs. (Then again, it's also rather easy to pedal the bike around without a motor. But I digress.)
Most European cities will make you carry your bike from time to time or make huge detours. But, like you, I'm not the target customer for this so maybe people in general are more careful with route planning.
