Often that's also because the programmer doesn't know how the optimizer will help them to remove inactive code also in C code. As a simple example, when I have a 'general' bulk-getter function in C which returns a large struct with tons of values but the caller is only interested in one value, the compiler will 'collapse' the entire function call to a single memory access (if it can see the function body, but this is where LTO comes in), e.g.:

https://www.godbolt.org/z/n3Y54Yhqr

This is basically the gist of C++ 'zero cost abstraction', but C-style (the bulk of what enables C++ zero-cost-abstraction doesn't happen up in the language, but down in the optimization passes).