Likewise, we've spent a ton of valuable time graphing and mining data from all over our stack; it's only natural that after that, we care about how we present that data. Like typography, it's hard because it's an arduous hunt for the aesthetics hidden behind information. But this hunt is what makes us human.
That's why we design beautiful graphs, just like we'd design a beautiful typeface. Not because we are GitHubbers, nor because we are designers, but because we are humans.
Life is too short to stare at ugly graphs. And yes, you should feel bad.
PS: we have no project managers at GitHub.
Just 'cause there's non-code aspects doesn't mean they have freedom to be ugly when you're striving for excellence.
edit: English fail.
I think it's because, when it looks good, it's easier to find the information, and it makes you want to look at it. It makes you want to work with it.
Now then again while I realize github has a huge, huge amount of things running at the same time, I do find github quite a bit slower than my "own" remote repos :P
As a developer I'm totally jealous of the ability to create simultaneously elegant and usable UI/UX, even when I used to try and just copy the CSS and structure right from the HTML source (I've since stopped) it was always missing that "je ne sais quois".