Because memory...is everything. Physically speaking, a memory is nothing but a specific combination of neurons firing together--a symphony of neural activity. But in actuality, it's the filter between us and reality. You think you're tasting this wine, hearing the words I'm saying, in the present, but there's no such thing. The neural impulses from your taste buds and your ears get transmitted to your brain, which processes them and dumps them into working memory--so by the time you know you're experiencing something, it's already in the past. Already a memory.