It's not that Windows doesn't have standard locations, it's that most developers continue to ignore them. Microsoft set up special folders
for the various data types all the way back on Windows 95/98, complete with environmental variables that always correctly point to those locations (even if the user has moved them to a different folder/drive). The associated documentation makes it very clear what sort of data goes in what special folder, but even now most developers don't bother following these standards properly unless they are forced to (e.g. for Games for Windows branding).
In the case of saved games there is a special folder for that (%USERPROFILE%\Saved Games) from Vista onwards, but because XP doesn't support it most games simply save to Documents; only a small number of games check the OS being used and then save in the appropriate location. A few poorly ported games even put saves in %APPDATA%.
Unfortunately the safest way is still to back up the entire %USERPROFILE% folder plus the individual save subfolders for games that save into the game's own folder.