it may be a rich mixture but most likely worn out piston rings as seems like oil is leaking into cylinders and i guess we know a method to cross check that by looking at the exhaust.
if that's ok, then your intake manifold mush be very oily and most likely because of bad pcv valve or lot of oil has leaked into the manifold resulting in plugs becoming very dirty.
second solution is remove the intake manifold and get it cleaned via kerosene oil and install a oil catch in line with pcv valve.
third and last, get the car turned to eliminate if car is running rich, but that does not results in spark plugs getting dirty so soon.
that's my opinion
