Do you think humans don't have perfect memory because it's hard to achieve and millions of years of evolution haven't been able to? Or because it's convenient to forget in order to prioritize the more important recent information?

It's obviously the latter, a system that 'remembers everything perfectly' is probably not optimal in most senses. Mortality is a property of both life and artificial systems, forcing the same retention policy on new information and old information probably does so at the expense of lifespan or stability.