Some help please

Why do the parent and child process on Unix not end up sharing the stack segment? that would be a fatal waste of physical memory to share the stack memory programmers can use threads, which are more lightweight, not processes they would interfere with each other, producing catastrophic errors (for the processes involved), such as when the child writes a local variable that in the parent stores the return address from a function sharing memory requires active intervention from the programmer, so the OS does not share anything between parent & child processes