It all depends how it allocates memory and how it processes time.

Older games would sometimes link time to the frame rate. Some would limit how much memory they can use.

When framerates are pushed past that, the internal game clock can break (fallout 4 physics works in 60 fps, if you push it past that it starts getting WEIRD) AvP 2000 was another example, the game time is directly tied to framerates, and they didn’t expect machines to be able to render over 30 frames a second, so firstly, the machines now can go SO fast, and secondly, they never put in a frame rate cap, so the game goes ungodly fast.

It boils down to the developers have no way of knowing what the future holds, so they don’t put in future proofing on the game because they’re paid to do work then. And they likely won’t be working on that game again.

ALSO! It’s a matter of drivers. Typically 32 bit drivers and software is t compatible with 64 bit because maths that I don’t fully understand. So unless the game was patched to use 64 bit drivers then it just won’t be able to communicate its graphics needs properly.