Because if it's faithful, then that would leave us with a movie where a mute protagonist solves puzzles and traverses through test chambers. Even the eponymous Portals have no deeper significance than a gameplay mechanic to solve those puzzles with. I say it would be a poor adaptation because there is nothing to adapt. There is one character (glados), no character arcs, discovering Ratman dens (a player driven action, mind) are the only non glados moments of storytelling, and the test chamber structure is entirely pointless when removed from a video game "win/lose" context.
Adaptation does not have to be literal to be effective, but when a play is adapted into a movie it likely uses a similar script, with similar characters, and similar songs. It resembles its original form in some way. A Portal movie, comparatively, would have to be almost entirely something original just because of the barebones source material.
You agree with me it doesn't have to be literal, but then you propose it has to be or then it doesn't resemble the source material anymore. You say you know adaptation is flexible, but then you qualify it as poor when the adaptation strays from a rigorous path.
I just disagree. Adaptation is not a 1 to 1 translation, it's a change from one art form into another art form and it's not judged in terms of good and bad just because it's close or not. In that process you use the tools of your medium to make up for the things you are losing (and you do it effectively if you understand both of them, you don't introduce a PoV shot into Doom and pretend the movie is a FPS), and with those tools you are allowed to give your main character a background, and arc, dialogue and depth. It will still resemble the original in some way, in the sense that the character goes through a process, is given a certain tool, discovers a different path and probably has a confrontation with a machine/AI that is in fact not quite what it seemed to be at first. It won't stop being "Portal" for that.
And then we can't forget that a "Portal" movie doesn't necessarily mean they are hinting at a 'Portal 1' story beat by beat, which is apparently what everyone is thinking for some reason. They've expanded the world, the characters and the story in Portal 2, and even if they choose to go a different way with different characters, it will still "resemble the original in some way" (your criteria, again), because if you are here trying to tell me that just because you don't have a mute main character it is not longer Portal, then you've missed the point entirely. There are many elements to Portal, I disagree that the source material is as barebones as you are hinting. Everything from the test chambers, to the AIs, to the turrets, to the facility, to the people behind the facility, to the materials to all the storytelling done around it and the gun contributes. Take most of those elements away, sure, maybe it turns into a different thing. Take one or a few and change them, it will still be "Portal".
I'm 100% down with this. JJ Abrams is a great director, and I have faith.
I really hope they do all of Portal though. Not just Portal 1. Portal 2 has (IMHO) some of the best dialogue in the medium and if they don't include that, I'll be bummed.
He's just producing, I think. But his influence should shine through the project nonetheless.