The other issue is that people are constantly telling others crap. People are constantly giving bad advice, making wrong predictions, etc. Sometimes these people are very accredited, very smart, and well-intentioned, they just happen to be wrong.

So when people receive advice, predictions, etc. they won't just accept, they use their own judgement. Which is also often wrong. But either way can be wrong, and people almost always trust themselves more than others.

The best thing you can do to convince a skeptic is show them very clearly or move on. The best thing a manager/lead can do to convince a skeptical employee of their business/design plan is show them very clearly or fire them if they don't follow the plan.