Ships charge WAY more for shoreX's...why?

When I read these posts about shore excursions it inevitably comes up that to book on one's own is less expensive than booking through the cruise line. My question is really this...... If the cruise line is gauranteeing the excursion companies large bookings, then the cruise lines should be getting WAY cheaper prices on the excursions. So why are the prices MUCH higher? Do the cruise lines do this strictly for revenue or do they believe they have extra ordinary amounts of processing they need to add into the cost? I have never been brave enough to book tours privately and have always gone with the cruiseline bookings. I have a sense of safety by doing this because of all the obvious reasons, ie: very organized, ship can't leave if you're late, etc. However, I always (since reading these postings) feel a bit ripped off on price too. Any feedback for me on this one?