Good games don't need sequels. Sequels aren't the same game, they're your chance to see the same basic mechanics but more awesome, and with more features that maybe they couldn't have put in the first time around, that's the basic idea behind it. Sometimes you just love something so much you don't want it to stop, but you can only play through the same level so many times. Sequels tend to get more funding as well.
The difference is, those games were all story driven. Portal was just a little gimmick with a story tacked on. And a game would need a sequel in order to carry on the story (Shenmue for example).
In light of that argument Super Mario Bros is also a gimmick.
And I totally disagree that a sequel is needed to carry on the story. There are a lot of sequels that have nothing to do with the story of there predecessor. For example Zelda and Final Fantasy. In a good game the story is always a finished product. If it is really necessary for the story to continue in a sequel, then the game is episodic. Something that Shenmue had as its main problem. Each part (of both) was sold as a full game, but as part 2 was released the creator already confirmed that it is 16 chapter story and each chapter is a game.
You found Portals story more entralling then Half-Life's? ... You were captivated by "Wake up in Cell, use portal gun to escape"?
I know it's a matter of opinion and what not but... I mean that's like being captivated by Spot the Dog as opposed to something along the lines of Lord of the Rings...
You do know that LotR can be abstracted to: "A group of adventures go to a far away land to destroy the weapon of the enemy, in order that the enemy can't use it to conquer the world and thus gets his ass kicked." It is all about how much detail you leave out.
Sequels aren't made because the story needs continuation, but because one or more of its predecessors were well received and earned the developer a lot of money. Why do you think that Fifa gets a new release each year, or that there are so !@#$% many games with the Pokemon franchise.
In all seriousness, what I don't understand is how upset you get over such an insignificant announcement.