Because bitmaps target a specific resolution they can be tweaked so that features are aligned to pixel boundaries resulting in less anti-aliasing.
Truetype fonts are an example of a vector format that can still look sharp at arbitrary resolutions because they can include hinting programs that align the control points to pixel boundaries. This is why fonts tend to look sharper on Windows and Linux than MacOS X. Linux and Windows default to strong hinting, and MacOS X uses only slight hinting, preferring accurate shapes to sharpness.
> Linux and Windows default to strong hinting, and MacOS X uses only slight hinting, preferring accurate shapes to sharpness.
I know you already know these things, but for the sake of discussion (or other people).
Linux allows you configure hinting. It's true that by default (depending on how you define default) you get no hinting, but no reasonable distribution that I know of has shipped with that default for the past 7 years. Quite the opposite in my experience. Linux fonts (thankfully) look more like OS X fonts than Windows fonts.
Unfortunately, this configuration usually happens in some Gnome/Unity/XFCE/KDE configuration format, so if you want to use a better window manager with no DE, you sometimes have to recreate this from scratch, although, as someone who obsesses over font rendering and doesn't like the DEs, this is easier now than in 2000.
And then you have Google Chrome who just ships with its own font rendering. Gah.
Truetype fonts are an example of a vector format that can still look sharp at arbitrary resolutions because they can include hinting programs that align the control points to pixel boundaries. This is why fonts tend to look sharper on Windows and Linux than MacOS X. Linux and Windows default to strong hinting, and MacOS X uses only slight hinting, preferring accurate shapes to sharpness.