I don't agree that they don't care about story and only do it for marketing.

I never said that, but sure, you're free to disagree with the thing I never said.

halflife's episodes are all about an attempt at continuing that story.

And, as I said earlier, they got bored, found it to not be a satisfying thing to do and stopped and never did it again. Episode 2 was 17 years ago. There will never be an episode 3 or half-life 3.

I think that the Cave and Glados bits of portal are a large part of what made those games

That part sold those games but funnily enough they aren't even half of the game. Most of Portal 2's content is on the multiplayer coop puzzles. They have more levels and a play through runs for more hours than the single player portion.

I think the only way to know would to be an insider

We have them, I'm not making shit up. There are dozens of interviews, documentaries, in-game commentary and books written by Valve staff themselves saying exactly what I have been summarizing in these comments. This idea isn't mine, I'm just repeating what people at Valve have publicly said about game development.