Rationality suggests incorporating new ideas into existing ones. Where are these people change their projections based on the slowing increase in computing power? (aka second derivative of computing power).
Please don't dismiss all ideas about AI, intelligence explosions, and superintelligence (e.g. http://singinst.org/upload/artificial-intelligence-risk.pdf ) just because some related concepts may be misguided.
(Not saying that you are being dismissive, just pointing out a distinction that you or others may not be aware of.)
So, a super intelligent Jupiter brain simply can't predict* the temperature of every cubic centimeter out to 5 decimal places 10 years from now regardless of what computational power it's given or what measurements it takes. And, peoples behavior is influenced by the weather so again it's can't model human behavior to that nth degree. Now that's just one example, but you really can't predict the stock market accurately over long time frames for the same reasons etc etc.
*again ignoring more direct influences.
PS: Now QM might not be correct, but assuming a world view without evidence is the realm of religion not reason.