And if you're a scientist (or anyone) in a leadership position communicating with the public, it's necessary to understand that you always speak with authority regardless of whether want it or actively disclaim it. If you tell people "this is what the science says", and you have a ton of authority, people will be more confident in your beliefs than you might want them to be.

The plus side of that in a pandemic scenario is that when you say something is necessary, people do it. The downside is you ruin the credibility of everyone you're leading if you're wrong.

Fauci isn't the average scientist. The cost-benefits to him speaking on an issue are different than a researcher coming up with a new theory on how COVID spreads.