Having recently been hit with some mmo grief, I can, at least in part, provide an answer as to "why" this happens. I think the first catalyst was the economic downturn which started in the latter half of the last decade and hit the entertainment industries before everything else. They're the easiest to shelve in times of austerity and it saw lots of small movie, tv and game studios go into bankruptcy or being bought over for much cheapness by their bigger brothers.
What we ended up with is a very polarised industry where some people did certain things very well. Enter Steam and downloadable games became "their thing". Microsoft went with XBox Live, Zynga owned the web platform and various other companies specialising in making lots of cash doing one specific thing. Then things changed.
We demanded more interaction between our platforms. YouTube that last teabagging in CoD? We have a plug in for that, want to compare scores in Team Fortress? We have an app for that. So the download content, development of in-game content and ability to weave these things in with other popular apps came into being, but dealing with seperate companies who all want to earn their slice of the pie. Enter the lawyers...
Since no-one trusts anyone when it comes to big business, the simplest thing to do is take a game and carve up the various elements of it and farm it out to the various companies that specialise in that section of gameplay. Since each is accountable to itself and whatever overarching company ties it all together, each have accounting mechanisms for their section and, due to data protection, have to have security around their slice of the pie. Thus we end up with multiple accounts, multiple varieties of password security (and therefore different passwords) and a lot of very unhappy gamers.
To tie this all together again would involve the industry standardising on their security and possibly having to reject certain partners for not keeping to the parent company's security standards. It's a LOT of money and a lot of work to sort these kinds of issues out. Been there, done that and really don't wanna do it again and it wasn't even a gaming company with millions of demanding customers!
One solution is for the parent company to provide a WAN which the subsidiary companies can tap into and centralise all the elements of a single game so that it's a single sign on for all game content, but that would mean the parent company would bear the brunt of the costs setting those up and we all know how generous AAA companies are...
Still, most of the contributors to The Escapist have made comments about this issue and the fan backlash is always severe, vaclising how unhappy you all are with the hassle involved in just getting to the game content. We can only hope that some bright spark in a AAA company wakes up and tries a new kind of partnership with all the pilot fish companies that feed off it's games and comes up with a solution that suits everyone. Sad to say, but I think this one if the hands of the lawyers to sort out...
