I can think of three explanations, but I'm not sure how likely they are: Gamma ray bursts are exploding unfriendly AGIs (i.e. there actually is astronomical evidence), unfriendly AGIs destroy themselves with high probability (lack of self-preservation drive) or interstellar space travel is impossible for some reason.
If interstellar travel (and astroengeneering) is impossible, that is enough to explain Great Filter without additional assumptions.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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