One possible explanation, why we as humans might be incapable of creating Strong AI without outside help:
As with all arguments against strong AI, there are a bunch of unintended consequences.
What prevents someone from, say, simulating a human brain on a computer, then simulating 1,000,000 human brains on a computer, then linking all their cortices with a high-bandwidth connection so that they effectively operate as a superpowered highly-integrated team?
Or carrying out the same feat with biological brains using nanotech?
In both cases, the natural limitations of the human brain have been transcended, and the chances of such objects engineering strong AI go up enormously. You would then have to explain, somehow, why no such extension of human brain capacity can break past the AI barrier.
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.