Have you ever noticed how directional road-signs always appear to be placed by people who already know how to get where you want to be? Or, if you are trying to navigate your ways round a business park or an industrial estate, that the signs are not as clear as you'd wish?
It's much the same with many websites. The person who created it knows how to navigate their way around it; and then forgets to make it as clear and simple as it should be for strangers to complete a journey from arrival to purchase or other desired action.
REMINDER: Make sure your website is user-friendly