Why doesn't GCC optimize structs?

gcc does not reorder the elements of a struct, because that would violate the C standard. Section 6.7.2.1 of the C99 standard states:

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared.

Structs are frequently used as representations of the packing order of binary file formats and network protocols. This would break if that were done. In addition, different compilers would optimize things differently and linking code together from both would be impossible. This simply isn't feasible.

GCC is smarter than most of us in producing machine code from our source code; however, I shiver if it was smarter than us in re-arranging our structs, since it's data that e.g. can be written to a file. A struct that starts with 4 chars and then has a 4 byte integer would be useless if read on another system where GCC decided that it should re-arrange the struct members.