I think that if they are in the right frame of mind, most people will make funeral arrangements. For those that don't, it may be due to an illness or they do not have the capacity to think these things through. Pre-planning takes decisions and thought, so if a person is not up for that it may not happen.
Case in point is my mother-in-law who for the past year was unable to make decisions due to an illness. The children did the preparations as her life drew to an end. Also I believe in some cultures, it may be bad luck to do so since some may believe, they can be cured.
There are many reasons why people do not pre-plan. Depending on their circumstances and support from family may depend on whether or not this happens.