Why exactly is WD-40 not a true lubricant?
WD-40 is composed of many different chemicals, and only about 10-20% of these are lubricants.  Even then this small amount is only a very light mineral lubricant.  Most of the other components are intended to help penetration, but are volatile and are intended to evaporate.

WD-40 does provide some longer term lubrication, but the problem is that the volatile components will dissolve and displace whatever lubricant already existed (e.g. grease), and then the majority of the WD-40 will evaporate.  So really it should not be used on anything that requires propper lubrication (or at least, a proper lubricant should be added again some time after using WD)