It is not very hard, but it is additional complexity, and it then requires the language to have a way to opt out so you can handle things like FFI or explicit o

It is not very hard, but it is additional complexity, and it then requires the language to have a way to opt out so you can handle things like FFI or explicit ordering (usually for padding to avoid false sharing and friends). So most languages opt to follow what their predecessors did: do nothing and task developers with reordering the structure if they want to minimise its size.