So it's in the best interest of those game developers to make sure older versions work too (especially now that Ubuntu desktop versions get 5 years of support).
It is correct that you could extract the libc6 package and put it in a dir which you specify using LD_LIBRARY_PATH but thats not really user friendly. Usually nobody ships libc6 with a game.
That's just a dark theory that this would work. Two examples:
a) Warsow - the binary in the tar.gz would work on U 10.04 but does not because it is compiled against a custom installed libpng15 which is not available in Debian/Ubuntu. Precompiled binaries for U 12.04 is available to be used on top.
This is compiled against a libc6 from U 12.04. It starts using an extracted libc6 i386 - very user friendly.
Basically it is not that problematic for those examples as you can get the source and compile it on your own. But it shows that Linux binaries are often problematic for ppl with older Ubuntu installs or other distros like Debian. It's mainly the missing out-the-box experience that you usually get with Win.