The generated code is not just a function of the *.plecs model file. Regarding the model itself, there can be dependencies on external code such as *.m files that are run as part of initialization or files that are included in C-Scripts.
The generated C code also is a strong function of Lua scripts which could be modified externally as well.
These are reasonable justifications for regenerating the code on every build, even if the *.plecs model itself remains unmodified.