Causation is something we infer, through empirical reasoning. When I am taking a math test, and see a math problem, then solve it, it is reasonable to infer that the reason I solved it, was my desire and intention to solve it. If I have a thought I want to share, then speak or write about it -- likewise, a mental event is reasonably inferred as causing a physical event. there are multiple examples we all experience every day in just navigating our lives -- the feeling of hunger, plus images of a sandwich, lead my feet to walk me to the kitchen.
The only reason that anyone questions this very evident fact about our world, is because of an ideology -- that of physicalism.
But the various physicalist alternative "explanations" of the only "apparent" relationship between mental events and physical events, have all been shown to violate one test case or another. And in general, they require some otherwise unknown and unevidenced force or relationship to tie the mental to some unknown and more central physical process, which is then postulated as the "real" cause of both the mental event, and the apparent physical response to it.
Our world IS a complex place, and there are a lot of subtle and non-intuitive features to it. That the mental COULD be derivative off the physical, and only "apparently" rather than actually causal, is certainly a possible outcome of investigation. But the "hard problem of consciousness" is that none of these physicalist explanations that have been proposed to date both pass all our test data about consciousness (See Blackmore's A Very Short Introduction to Consciousness for multiple failed test cases for physicalist speculations), AND would then lead to our being conscious. elaborating on this second point: in almost all cases for these physicalist speculations, the consequence of evolutionary variance acting on a non-causal consciousness would lead to consciousness disappearing.
The "hard problem of consciousness" is that these physicalist speculations are less straightforward than just accepting the causality of thoughts, they contradict observations we have from consciousness studies, and they do not explain why we have consciousness at all.
Note, contrary to Blackmore, there are possible "physicalist" explanations if one allows for causal emergent consciousness to be a feature of the physical. This would involve accepting that thoughts really are causal.