Sounds like a total failure to document anything....
Everywhere I've worked in the past few jobs I've had, processes and procedures and anything important about business process has to be documented on a wiki (or more recently, everyone seems to have gone to confluence) and documenation is considered REALLY important.
Sounds like the places I've worked have learned the value of employee knowledge and suffered from employees leaving with vital knowledge not documented.
You either work for either a really tiny organisation, or a business that just hasn't suffered through an important person with a lot of important knowledge leaving.
Oh well. Their problem.
Move on and forget your current employer.