Eli5: Why do we forget things but people with eidedic memory can remember everything?

Question

I can forget what I studied for an exam, but others can remember. Then you have people with eidedic memory that don’t forget at all.

In: Biology

Answer 1

One of the primaries directives of all human organs is to consistently reduce unnecessary effort, in order to optimize energy consumption. The brain is no exception, so it’s important for a healthy brain to routinely cut out low-priority information. We only forget things because the brain is designed to optimize for thoughts that either happen frequently, or are considered a high priority for survival.
So someone that doesn’t forget things might have been at a disadvantage in the scarce hunter-gatherer world our species originally evolved in, simply because their brain consumed too many calories, or because it optimized for redundant memorization rather than survival.