Since every day and situation is different, every person needs piles and piles of advice and techniques how to properly serve Hashem or even to get through day to day life. Unfortunately there are many people that currently follow Mefursamim shel Sheker. Even if they frequently consult with their Mefursom, it is very unlikely that he can give them even a minor part of the advice needed to get through life. Even if their Mefursom wrote a book that is always accessible to them, it still does not contain even a fraction of the advice needed. Even if their Mefursam wrote many books, they still will not get close to covering the information needed for survival. Even if their Mefursam belongs to a large dynasty that existed for many years and produced many texts, it still will not give them the advice needed for all areas of their life.
Even if a person does not follow any Mefursamim but relies on his own perception of earlier texts from true Tzadikim, he is still not covered. He will be confused by many outdated techniques and contradiction in between the various paths given by past Tzadikim.
With Nanach it is all easy. Rebbe Nachman gives advice for anything and everything a person will ever need. There is no confusion at all, if Rebbe Nachman say it, we do it. If Rebbe Nachman did not say it, we do not have to do it.