That doesn't explain why the universe isn't filled with strong AIs, however...
The anthropic principle selects certain universes out of all possible ones. In this case, we can only exist in the subset of them which admits humans but prohibits strong AI. You have to first subscribe to a version of many worlds to apply it, not sure if you do. Whether the idea of anthropic selection is a useful one still remains to be seen.
If Strong AI turns out to not be possible, what are our best expectations today as to why?
I'm thinking of trying myself at writing a sci-fi story, do you think exploring this idea has positive utility? I'm not sure myself: it looks like the idea that intelligence explosion is a possibility could use more public exposure, as it is.
I wanted to include a popular meme image macro here, but decided against it. I can't help it: every time I think "what if", I think of this guy.