Forking a process on a modern Linux is relatively cheap these days, and for many jobs you will just prefork all the workers anyway. Passing messages via thread mailboxes or IPC pipes are about equally complex endeavours.

If threads had never been invented, I suspect that 90% of modern multicore programs would have done just fine on a mixture of multi-processing and asynchronous I/O. (The other 10% would have done poorly, though.)