I might have an answer to your "why" question.
In the old days, yeah I'm old enough to have experienced them, there was a school of thought that you remove the factory plugs, screw in a zerk, grease the item, remove the zerk and put the plug back in place. Not many people bought into that theory, but some did. This might explain it, but regardless, if the machine were mine, I would remove the plug, put a zerk in its place and keep it greased.
I just got through rebuilding the spindles due to the previous owner never greased them. Keep them greased about every 25 or 30 hours and you will go a LONG time before rebuilding the spindles.
My $0.02,
Larry