It could be a combination of bad luck, long miles, no fenders, and as you note, lack of seals. In a case like yours, I might take a chance and drop oil into the BB and hubs and see if that flushes the grit out. Sometimes it does. It did with my Campagnolo Record BB and hubs from the early 80s. The BB cups had threaded holes where the spindle went, designed to push grit out. The hubs had oil ports in the centers so the oil would go from center to axle ends. I did this on my three month European tour which had a lot of rain. When I got back home, my bearings were stunningly clean because of my frequent oiling. I might have also oiled my headset, but I don't remember clearly. I didn't have fenders on that trip.