If it really is reasonable to place a large amount of probability mass 15 years into the future, why are virtually all mainstream scientists (including the best ones) apparently oblivious to this?

How do you support this? Have you done a poll of mainstream scientists (or better yet - the 'best' ones)? I haven't seen a poll exactly, but when IEEE ran a special on the Singularity, the opinions were divided almost 50/50. It's also important to note that the IEEE editor was against the Singularity-hypothesis - if I remember correctly, so there may be some bias there.

And whose opinions should we count exactly? Do we value the opinions of historians, economists, psychologists, chemists, geologists, astronomers, etc etc as much as we value the opinions of neuroscientists, computer scientists, and engineers?

I'd actually guess that at this point in time, a significant chunk of the intelligence of say Silicon Valley believes that the default Kurzweil/Moravec view is correct - AGI will arrive around when Moore's law makes it so.

200 years? There is wisdom in some skepticism, but that seems excessive. If you hold such a view, you should analyze it with respect to its fundamental support based on a predictive technological roadmap - not a general poll of scientists.

The semiconductor industry predicts it's own future pretty accurately, but they don't invite biologists, philosophers or mathematicians to those meetings. Their roadmap and moore's law in general is the most relevant for predicting AGI.

I base my own internal estimate on my own knowledge of the relevant fields - partly because this is so interesting and important that one should spend time investigating it.

I honestly suspect that most people who reject the possibility of near-term AGI have some deeper philosophical rejection.

If you are a materialist then intelligence is just another algorithm - something the brain does, and something we can build. It is an engineering problem and subject to the same future planning that we use for other engineering challenges.