Job control is what happens when you press ^Z to suspend a program, or when you start a program in the background using &

Typically, a process associated with a controlling terminal is foreground process and its process group is called foreground process group. When you start a process from the command line, it's a foreground process:

The shell process itself is in yet another process group all of its own and so doesn't receive the signal when one of those process groups is in the foreground. It's that simple.

Switching "jobs" between foreground and background is (some details aside) a matter of the shell telling the terminal which process group is now the foreground one.