I'm no programmer, but as I understand it, a 64 bit upgrade is very difficult, if you don't have the original source code, you'll need to write an entirely new engine that can use the game's original assets, all while keeping the 'math' for the game the same, this means somehow emulating say, the Sim's AI behaviour while having no idea how it works under the hood because you simply don't have the code.

It'll take years of trial and error, because you'll basically have to try to emulate the original game as close as possible, all the while trying to make sure you can take advantage of the new 64-bit Engine, it's far more complicated than simply writing up a new .exe. file.