If we achieve runaway AI, the stock market goes to infinity. So from an expected value standpoint, massive spending on it is worthwhile. Even if the odds are tiny, the payoff warrants a massive bet size.
You underestimate the non-computerized part of humanity. Even with automated plants building other automated plants the propagation speed will be highly constrained by natural factors, plain unavailability of resources, and xenophobic nature of humanity of course.