How much memory capacity does a tree have? If you punch a hole into the tree the tree stores the information of that punch a long time into the future by keeping the hole. It doesn't really make sense to quantify the amount of information that the tree can store.
Evolution didn't design humans to be good at recalling past events. It designed us to be good at doing things. Our brains are optimized to learn to deal with the world around us.
Learning a skill like walking means having to gather a lot of information about how to balance your weight. It's not useful to be able to remember every experience. The same goes for most information storage in the brain.
Barbara Arrowsmith-Young was a woman who had a learning disability as a child.
I reversed numbers and letters, struggled with reading and writing, and could make no sense of the relationship between the big and little hands of an analogue clock. Asked to perform the simple addition of a small two-digit column of numbers, I would randomly choose numbers from the left or right side. The logic of basic math, the concept of telling time, the ability to truly comprehend what I was hearing or reading: all eluded me.
On the other hand she had a verbatim auditory memory and a visual photographic memory. It seems that those people who can consciously remember all the details don't manage to be good at recognising the relationships between the details.
Understanding the relationships between details is important to make good decisions. If you understand that a mushroom is poisonous you don't need to remember consciously the time where you throw up when you eat the mushroom.
As most stored information is not stored to be consciously recalled it's difficult to count that information in a meaningful way.