Because it is non trivial when using C or any derived language that has copy-paste semantics with C.

It starts by C not having fixed sizes for its datatypes.

Sure there were always macros/typedefs with such fixed sizes and C99 introduced stdint header.

However not everyone actually uses them.

Then there are the bit fiddling algorithms, unions and casts that assume a specific memory layout.

Followed by code that might actually become UB when switching to another memory model.

All of that scattered across hundreds of files, not written by a single person, with an history of decades of code changes.