I just dont get why you have to assume that though?

Maybe I'm a pessimist, but I've met and worked with enough humans that I think the best assumtion is that they're all full of shit until they prove otherwise.

It's fine to rely on experts for some things, but if those experts aren't subject to independent scrutiny or directly independent of the claim or sunjecy under test, or can't give clear testable /replicable evidence, I'd just not put much weight on their testimony as a source of evidence.