If you put grafiti somewhere which is hard to get to it is less likely for others to destroy the grafiti and therefore it is more likely to stay up for a long time. A lot of the grafiti is done at night when it is harder to get caught and also easier to do invasive things like blocking sidewalks or roads. A lot of the more serious grafiti artists will secure themselves to the best of their efforts with safety harnesses and spotters. They will use properly secured platforms to paint from. If you drive around at night you might confuse them for legitimate maintainance workers.

Less serious grafiti artists might not do all of these safety measures. But they will still bring the tools required for the job and usually a couple of friends. Accidents still happen but not as frequent as you might think.