Probably because dynamically-linked binaries are smaller, use less memory (by avoiding duplication), and can get fixes or security updates without rebuilding or re-deploying. When you're a distro it hardly makes sense to ship a copy of libc inside every single binary. Any fix/update to libc would require re-downloading basically the whole system!
It's getting even stranger in the world of free games. When games need library fixes (very typical situation as games and engines are very interwoven), which are not yet in a distro (and won't be for some time because the library is for example not yet officially updated or maybe a certain patch simply won't be included). On a system like Windows it's no problem - modify the library source and add the dll. On Linux distro's... well, just not easy possible. Which means funny enough that it's easier to modify library sources in the in the proprietary microsoft world than in the free software world. We got freedom coming with library + distro gatekeepers so to say... which pretty much sucks (even for the library authors).
"Dynamic linking considered harmful": http://harmful.cat-v.org/software/dynamic-linking/ http://harmful.cat-v.org/software/dynamic-linking/versioned-...
OTOH, arguments of the other side, for proper polemics: "Static linking considered harmful": http://www.akkadia.org/drepper/no_static_linking.html
I think the days when it mattered are really coming to an end personally.
The amount of space (memory/disk/bandwidth) taken up by binary code is minuscule compared with data/video/streaming/games/etc etc