With saltwater being such a corrosive agent, bearings in reels designed specifically for saltwater usage need to be tough and able to handle the torture. Despite how good they are built, they are still the weaker links in a saltwater reel.
Hence reel engineers place them only in places where they are most required, and areas where they are least affected by saltwater. Rings and washers are used in place of bearings in areas that are most susceptible to saltwater damage.