Why is it in someones interests to believe (say) that science is all a conspiracy.

Firstly, they enjoy the drama. Worrying that the world is going to end makes life more exciting than the day job.

Secondly it 'proves' that they're not really as stupid and powerless as science makes them feel.

Also, it's very rare for hardcore CT followers to be even slightly literate in basic science. Facts and paranoia look indistinguishable to them, because they don't have the background to tell them apart.

See this thread for a depressing example.