With that being said, being an Atheist doesn't automatically make a person rational.
Being rational 'opens a person's eyes' to the beauty of the universe.
I don't think true rationality is something that happens naturally. Brains are easily deceived. Critical thinking is needed to be rational.
A person has to dedicate a lot of time & energy to find out what is real and how things actually work in real-life.
A person doesn't need to know everything to be an Atheist. Meaning a person doesn't have to know everything to disbelieve in a god or gods. Atheists just go one god further.
It's hard for a lot of people to take that one step further, so they stay an agnostic theist instead of atheist.