it's because they don't know what good sources are or how to analyze them. it seems like a trivial matter but it's not. people in general put more trust in sources they agree with, whether they are valid or not. people in general don't like data and complex interpretation. even very smart people, i think, with many exceptions of course.
Unfortunately that's very true. People tend to only look for information that 'proves' what they already think. Not everyone is a scientist. I remember a Belgian scientist on tv literally crying with joy because data he researched disproved a theory he had thought for thirty years to be true. I'd never seen anyone being so happy to be wrong. :-)