Human brain is superior than till-date computers, yet why we perform poor than computer?
Three reasons:

Most of the storage capacity of the brain is used to store implicit knowledge. For example how to move your body, how to automatically recognise objects or how to hear and parse your language. These things may seem easy to you, but they require a lot of learning. Just look at how clumsy five year olds still are, they have been training how to use their body for five years straight and still skin their knees all the time. 

Humans are actually able to retain huge amounts of information, both in their short term memory and in their long term memory. We know this because there are savants and hyperthymesia. It is likely that these abilities are actively suppressed in neurotypical people.

Because it is just not evolutionary adaptive to remember every detail. Intelligence depends on weeding out unnecessary information. If normal people learn huge amounts of information they often rely on memory abilities that are evolutionary adaptive, like the ability to remember a path from landmark to landmark.