I think the answer is that they feel that because we know how individual particles (such as electrons) interact, then they 'know' how human brains work!
That form of knowing isn't much good in practice, because the equations become more and more complicated as you add more particles, and so even the equations for small molecules can only be solved imprecisely.
However, they are also extrapolating massively. There are approximately 10^23 molecule in one gram mole of matter, so it is a huge assumption that what works with one or two particles may not work in human brains! Not all physicists think this way, see for example the work of Henry Stapp.
David