It's because when gameplay, storyline, and presentation of games like Baldurs Gate 2, Planescape Torment, and other classic 90s games are infinitely superior (and I do mean infinitely superior) than that of modern day games, you begin to wonder what is wrong with the companies that are making RPG games.
The exact same thing wrong with DC/Marvel comic books today.
A good story has a good author. AN author. I might not agree with everything the author says or does, but there is an author, and they have a story to tell.
A bad story has a committee of businessmen trying to decide what "the youth today wants" to put placed into their "franchise" (not story, franchise).
Rather then creating new original content, old stories get rebooted again and again (spiderman for example has over 41 separate continuities). I can say with assurance that if <insert your favorite book> got rebooted 41 times it would be crap.
Beyond the mere storytelling, gameplay is being packaged into "what the stupid sheep wants" (according to the belief of business execs who don't actually like games), and further influences by marking (lets put an in game quest that links you to a real world store where you can buy that content with real money!... ex: dragon age, fable 2... thanks for raping immersion!)
I think planescape torment was able to be such a good game because it was a product of love. That being said, its also comparing the best of the best of a whole decade to whatever you have now. Not every game can be a hit... and just love is far from enough. The game has to be well designed, with fun intelligent gameplay, etc. And planescape, as awesome as it was, is far from perfect.
Looking at those older games that are held up to such high esteem. They had their gameplay flaws... where they excelled is in the delivery. Amazing dialog, story, and themes that sucked you in and created unparalleled immersion... you weren't some dude playing a game, you were a hero on a quest.
Modern games often slap together some generic quests, no impeccably written story, no amazing dialog, and horrid voice acting... Ah the voice acting. One of the biggest downfall of modern games is voice acting. When it became practical to have voiced over everything a lot of companies did so... to save money MOST games have the programmers and their immediate family do the voice overs (yes, really). This is TERRIBLE and makes for an inferior experience to a voiceless game. A picture of a person with appropriate background music and text color lets your imagination fill in the voice properly. No gratingly bad delivery of teeth gratingly bad dialog to tear you out of your immersion.
To be honest, earlier bioware games had poor voice acting. but starting with jade empire it became really good. Mass effect and dragon age featured top notch voice acting. No stupid "false choice" between puppy torturing and stupid over-altruism (although they still had their moments, and jade empire had nothing but those). With dragon age the "karma meter" was done away with entirely to be replaced with "consequences"... you do something because you want to, and the reward is changes in the world... people die, get saved, get transformed, get punished... your actions have a consequence rather then merely adjusting your karma meter.
Mass effect had flaws, but I think it still stands as possibly the best RPG of all time.
Dragon age on the other hand, had such an atrociously bad game mechanics, that even the awesome interactive movie could not compensate for it. No amount of perfect dialog, characterization, character development, and epic voice acting and story could drive me to actually finish the game... because the gameplay itself was that of a really really bad MMORPG.
Finally, there is the issue of quality control... the video game industry crashed due to bad quality control in the 1980s. Most companies went out of business, and they swore "never again"... everyone forgot that and we are back at it. Nowadays 9/10 PC games can't be played to the end do to a 100% reproduce-able show stopping bug that needs to be patched (Showing utter lack of playtesting... example, Neverwinter nights 2 main campaign got stuck when you meet the elf druid, always! they simply haven't bothered playing it even once), and even then they are buggy.
Modern console games are only slightly better, and getting worse by the year. ALL current gen consoles can patch games over the internet. So the majority of console games are released buggy and unfinished, then never fixed.
One of the things that made portal stand out so much is the insane amount of quality control they did. Not only did they find and fix all bugs before releasing it. They PLAY TESTED! And they did so A LOT... listen to the commentary and you will find that in area after area they made sweeping changes due to player responses to play testing, then they did so again, and again.
http://www.youtube.com/watch?v=o1kvadcJwmE