Static file descriptors were a bit more common in the old days, but look horribly out of place in modern code. Keeping the code fork safe is easier than keeping it thread safe, at least with fork you aren't sharing the heap.
But you're sharing file descriptors, which might be for devices, or for SOCK_SEQ connections, etc, and you can't just have the parent and child step all over each other writing to them. Now, you wouldn't do that, but you might use a library that lets you end up doing that without noticing. Fork-safety is not trivial.
