I suspect the most outrageous liars are the ones who ostentatiously declare how willing they are to be "under oath." It's like watching a child declare "Check -- I'm not crossing my fingers!"
I recall hearing this aphorism as a child and it has always stuck with me: "The Lord loves to make liars of us all," which I've generally interpreted that the only reason we make _noise_ about how "trustworthy" we are is because we are trying to convince someone to the contrary of the evidence they see.
It's not that I have lost faith in the ability of humans to not make fools of themselves, it's just that I've come to realize that for every social construct upon which we mutually agree may be good for us as a society to follow, there are always scoundrels who see it as a way to hide their perfidy, and there are always those who cannot violate the pretend sacrosanct structure enough to scour out the shitbirds.
There are things about this species that annoy me, and a willingness to allow these kinds of excuses is one of them.