Because usually when we look for something, we wind up looking in more than one place. And we do stop when we find what we seek.
So: You lose something. You look in the first logical place, not there. Check another, not there. Check the third, OK here it is. “It’s in the last place I looked”.
You lose something. Look in one place, there it is. “It’s in the last place I looked”.
No matter what, it is going to be ‘in the last place you look’.
Every now and then, the first place you look will have your lost object, but it will still also be ‘in the last place you look’.