Convenience and impermanence. But look at the size of that keyboard! And her happy smile! This is one of the issues that's ruefully discussed in some of my SILS classes, particularly the digital archiving and electronic records courses. It's become one of those burdens we've chosen to shoulder, I think, without really examining why we do it in the first place. Or rather, we propose lots of solutions as we try to understand the problem, which is likely not a technological one at all.
I love homilies and rules of thumb, and this Zhurnaly page collects a great set from Physicist David Stearn. It traverses the small (write yourself notes and index them) to the large ("Being a physicist is a great privilege. Be worthy of it. Most of humanity spends its life doing boring repetitive tasks."). Here's a slightly different version by Stern from his web page.