system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: if our brains can technically store almost every experience, why do we forget so much?

I've read somewhere that we relate our brains to have something like 2.5 pentabytes of "storage"

If this is the case, then why are we so liable to forgetting things like text, words, or memories?
gpt
The very best answer is simply: we aren't sure. We don't know exactly what the mechanism is for generally forgetting memories over time is, or how memories are transferred into long term memory.

We know that humans are capable of incredible feats of memory, especially in the cases of savants. 

Chances are that there is only so much evolutionary pressure to remember and retain details.

Lets take a simple example. You are walking through the forest for twenty minutes, There are branches all around, trees, flowers, bushes. In the long grass by the water you smell a strange smell, and a wolf leaps out and attacks you, but you escape into a nearby tree.

Now, what evolutionary pressures are there for retaining memories. A human who can instantly recall that memory of being attacked when they smell that strange smell, over time, will be less likely to die. There is some pressure to remember and learn from how we escaped from a predator. There is no pressure to remember walking idly through the forest, except perhaps remembering roughly the path in order to find water/food again. There is no or little pressure yet to remember the smell of each flower and bush either. A human who can recall how many leaves are on each tree or which notes each bird sang has no advantage over one who doesn't, and so there is no reason for that ability in memory retention to be selected for.

As time went on we probably selected for a greater and greater ability to remember (which plants are poisonous -> how to make items and retain taught information -> what each person in the community likes, does, and has said -> current needs).

But there would still be a threshhold beyond which improvements to memory gave no evolutionary benefit. Perfect memory still wouldn't be selected for. Would being able to memorize every detail of an entire area by looking at it once, recalling how many trees are on any route they've taken in the last 40 years help a human survive enough to be selected for? Even today having a bad memory isn't a good trait, but IMO it isn't so bad it is really being selected against.

Would being able to recall every, or even most, conversations and emotions you've felt decades ago actually help a human reproduce, in a significant way for thousands of years?

Memory, like everything else about us, has developed over a long time from environmental pressures. A perfect memory remains disadvantageous, so while that ability presents itself in the occasional human, it doesn't seem to stick around and be selected for.

**TL;DR: Our memory only got as good as our environment required, and our environment has never required a perfect memory.**