The main reason is simply convenience. There isn't always a MF dealer close by, but there usually is a NAPA close by.
My MF local dealer gave up the brand. So, I am looking at a 70 minute round trip to the next one. NAPA is like 5 minutes away.
If NAPA would have given us a verified cross reference, I would have gladly gone 5 minutes down the road and bought it for the same money, let alone when you consider adding to any savings, gas and vehicle costs. Then there is the fact that NAPA filters are often better quality than the OEM.
In the meantime, yes, I will have to plan ahead, order it online, and wait for it.
If MF had priced their filter more fairly, or could offer something NAPA or Baldwin could not, I would less inclined to seek alternatives. At $42, it's clear their taking advantage of me. So, there is principal involved too.