You can absolutely save data like that, it's just that it's a terrible idea. There are obvious portability concerns issues: little-endian vs. big endian, 32-bit vs. 64-bit, struct padding, etc.

Essentially, this system works great if you know the exact hardware and compiler toolchain, and you never expect to upgrade it with things that might break memory layout. Obviously this does not hold for Word: it was written originally in a 32-bit world and now we live in a 64-bit one, MSVC has been upgraded many times, etc. There's also address space concern: if you embed your pointers, are you SURE that you're always going to be able to load them in the same place in the address space?

The overhead of deserialization is very small with a properly written file format, it's nowhere near worth the sacrifice in portability. This is not why Word is slow.