It’s because most athiests aren’t really athiests at all, per the definition of not believing in God. They wholly belive in God, they are just against Him. A better, more descriptive, term would be anti-thiests.
As for agnostics, I claimed to be such for quite a time. Most of all, I was misguided with anger.
Just like Satan and the fallen angels that serve him. Probably more than anyone else, they know that God exists. But they have chosen to separate themselves from Him.
I was a secular humanist for a time, but that was driven by a combination of despair and egotism. I still struggle with the problems that come of having an outrageously large ego, but have made some progress in keeping that under control, as well as regaining a measure of hope and faith.