The scientific consensus is that no one ever fills up their memory capacity by learning facts and so on, even in the information age. My question is therefore, is there an evolutionary reason as to why we have evolved to have such large memory capacities, when we never come close to filling it up?
We don't have large memory capacities.
If anything I would have asked the opposite, why is our memory so bad and unreliable considering how large our brains are. Even with this gigantic brain we struggle to remember a few phone numbers. Here's how our short term memory compares to monkeys: https://www.youtube.com/watch?v=zJAH4ZJBiN8. I have no explanation as to why humans are so bad at this, this amount of data should be nothing for a brain of our size.
As for long term memory, I'm not really convinced it is that large really, it seems to me people in general in ten years.. have forgotten pretty much everything. Occational glitch.. occational stuff, and even that is usually vague and full of wrongs. And when people cram for exams.. take the best students ten years later and see how much they still remember, it will be just a tiny fraction.
What do you mean by "filling up their memory capacity"? Are you expecting some sort of a state where the persons hard disk is full and nothing.. nothing new can be learned? That state doesn't make any sense, the brain is always deleting from memory plus it would make more sense to have the memory slowly deteriorate, rather than fall off a cliff into no functionality at all.
We don't remember anything. We always feel like we are going to remember the now for so long and all but we don't. And we have this bias because we don't remember what we don't remember, which is nearly everything.