Ah, so your question is why pay full price for this type of product instead of less money? Because many people don't know about sharing the cost. Or they don't want to hassle with sharing. Or they don't want other people seeing their photos. Or they have plenty of money and don't care about the cost.
Or, if you are asking why accept this level of product? Maybe people have lower standards, they realize you can't spend a long time getting dozens of shots while other people are waiting their turn (unlike a photo studio). Or they want the magic shots, on-ride shots, videos, and standard WDW stills you can't get with your own camera. Or they want convenience.
Maybe it's some combination of those. After doing a shared MM for just the two of us, and how that worked out, I can see how easy it is to accept the price for the process/product - for a different situation (e.g. larger group, grandkids, etc).