One of the big reasons is that GOP control congress atm. The jackasses who flouted their responsibilities and legal obligations to answer to congress (under this administration and Biden’s) have been republicans. They knew that Dems didn’t have the stomach and/or the numbers to prosecute them or hold them accountable for evading their responsibilities - so the repubs called to give evidence or testify in front of congress just did what they wanted with impunity.

Fauci is a persona non-grata with this administration and its party as a whole. Fauci knows he’ll be subjected to any and all prosecutorial avenues the GOP can utilise. I believe the thinking is this is the surest way to protect himself. His legal way to say nothing without lying, maintain his own ethics and morals, abide by the law and his obligations under it, and ensure he doesn’t give them one iota of ammo to use against him in future actions.

TL;DR: Fauci has integrity and believes in the rule of law.