Comment by Animats

Because "fork" was easy to implement in UNIX on the PDP-11.

The original implementation was for a machine with very limited memory. So fork worked by swapping out the process. But then, instead of releasing the in-memory copy, the kernel duplicated the process table entry. So there were now two copies of the process, one in memory and one swapped out. Both were runnable, even if there wasn't enough memory for both to fit at once. Both executed onward from there.

And that's why "fork" exists. It was a cram job to fit in a machine with a small address space.