Comment by JoeAltmaier

Add to that, fork is (was) very inefficient. You had to duplicate the entire process state (page tables etc). Then the damn program would exec(), and you would tear it all down again. Took 100ms on older computers. Complete waste.

We would resort to making a weak copy, with page tables faulting in only if you used them. A lot of drama, so the user could make a goofy call that they didn't really want most of the time.