Pointers don't have focus. Moving your printer always causes the pointer to move, and clicking buttons always causes a click at the location of the pointer. Keyboards have focus, because typing a key causes the key to be sent to some window. Pointers also have the association with keyboards so that, when you click the button or something, you can be holding down Ctrl.
If you wanted, you could have two pointers, where the mouse controls one pointer and the trackpad controls the other pointer, and the keyboard sends key presses to the application that the mouse has selected, but the trackpad can only point and click.
Note that you can have more than one physical device that moves a single pointer or issues key presses for a single keyboard, which is what people more frequently want.