I gave it some thought and the only solution I could come up with to the question is that a lot of people just don't have the time to invest in a proper regular game of D&D. 

A lot of us are in our 30's , 40's etc have a bunch of commitments and responsibilities and can't invest the time in forming a group and playing regularly. (I'm not one of them, but only because I convinced my wife to give it a go).

That plus the whole money thing again.. It's a lot easier to hold a tournament and control things if games are structured to fit in slots. Makes it easier for businesses with regular opening hours to schedule games so gamestores can host and set fees for a 4 hour session.

It also means you can just bring a character to any game and sit in a one time slot. Never have to face those players again. 

To be honest I don't care for any of it, it's definately convenience gaming and though I understand it it's one of the things that put me off the meetup group somewhat. If it was all RPGA I doubt I'd be playing, fortunately Russel Vance (the home made miniatures man) was at my first meetup group, I sat at the table and he was doing his own thing with whoever walked in. The game went well and we met up the following day and have done just about every sunday since.

I can't understand using the same modules either. Doesn't really suit me from a player or once having been a DM perspective. It would be like reading the same book over and over or the same movie. If the story and plot stays the same, and each time you have new players that don't know one another, how do you build a team that can work creatively together at solving problems and getting through situations. Seems like everyone will be playing for themselves and collecting what loot they can find to build up their stats and loot for later games.