When you think about it, fictional authors have to keep track of a lot of stuff: details about our characters, what they are doing, thinking, feeling, their quirks, what they love, and how they feel about other characters. In other words, picture your own life, times ten.
What makes things even more tricky is when you throw complex characters into the mix. Unfortunately, not all authors do, and that's why some books fall flat. We, as humans, are complex, so we most relate to characters who are the same. We don't always understand ourselves, and sometimes, we also don't understand our characters.
Keeping track of everything can be tricky. I have notes about each character, but sometimes I miss important details and have to backtrack. For example, I have one character who has celiac disease. Every time this character sits down to eat with the group, I try to make sure there's gluten-free food mentioned.
I think you have to be dedicated to your craft if you write fiction. Your mind has to be traveling in several directions at once and, it can be exhausting. Someone recently pointed out that this might explain why I'm so tired after writing. There are days when I literally could take a midmorning nap after an early morning writing session. It might also explain why I can't keep track of my own life. Who has the time?