Not all ignorance is deliberate. Almost none of it is -- ignorance is the default.
Some things we don't know yet. Others have effect sizes so small distinguishing them from zero is a true and deep challenge, even with tremendous study done. Some things actually do have effect sizes of zero, and no amount of evidence will stop people from demanding more study.
And also, Any amount of study does not say much if all of them uses flawed methodologies ("but that is the best we have" does not make it any better), which has been a serious issue lately..
Some things we don't know yet. Others have effect sizes so small distinguishing them from zero is a true and deep challenge, even with tremendous study done. Some things actually do have effect sizes of zero, and no amount of evidence will stop people from demanding more study.