Users don’t know the terminology to describe the problem they have or to know what to look for.
Users haven’t studied the problem long enough.
Users don’t recognize details or signs which might aid in understanding the problem.
Users might not have easy access to the documentation, may not be qualified to understand it (because of language barriers or technical level), or they may simply not have the time or energy to use it.
Users might be unaware of the status of their computer/account/browser and/or they might be limited in their ability to obtain this information.
Users might have received incorrect or misleading information from someone else, or they might have made incorrect assumptions about the product.
Users may be familiar with one kind of product and lack the appropriate mental model for knowing how the product is supposed to work.
Users might have previous problems in the past and found it easier just to call technical support than to risk aggravating the problem when trying to fix it.