They are quite literally agents and manifestations of opposed cosmic powers. The clash between Light and Void is literally the starting point of the entire setting post-Chronicle. The very touch of the two has them blasted apart and repelled. They're thematically in complete opposition, hope vs. despair, and endgoal of stasis vs no endgoal, contentment vs. rage, one vision vs. many, etc. The Lightforged were for 25k years not only infused by the raw essence of this cosmic power but their leader considered the void ground for execution and destruction and opposed all learning of it. They have not built their entire society around honor and loyalty, they've built it around the Light and the Light has these as its consequence within a narrow field. Though even using the word society shows another problem of the Lightforged namely that being essentially at war 24/7 for millenia and having no civilians in no way informs their characterization. They've no reason to consider the void anything but the ultimate evil and even if they had such a reason, their nature as beings of the Light should render them incapable of acting upon such reasons as both sides are total in their being. Least of all to suborn their issues with the void to their loyalty to a faction on a different planet that they've been a part of for months at best at the time this happens.
Now, you are right that the Lightforged are a complete carfire long before we get to that point. Forgiving Illidan and the demon hunters and forgetting that a demon fried their leader apropo of nothing being one. Everyone being fine with Alleria except Xe'ra being another. A human and an elf who've been around them for 1k years, less than 4% of their existence, being their leadership and being able to upend their entire relationship with the power they're forged by because they're fucking is the peak however. What should be executors of the will of a cosmic power who will make everyone peaceful and prevent conflict by force if need be and accept no deviation from this are instead just draenei with a handful more skin tones. To concur with @
Kyphael, The Lightbound are what the Lightforged should have been and error 404 is what the void elves should've been. Neither should've been playable as they turn any future cosmic conflict into a milquetoast 'everyone is fine with everything provided it's in moderation' and nobody minding that there's an entire subset who have the outer gods whispering in their heads to kill their families 24/7 and are basically vessels of cosmic entropy.