How do I have an element follow the mouse without jQuery

I have this but it doesn't work

var box = document.getElementById("box");

window.addEventListener('mousemove', function(e){
    var left = e.pageX;
    box.style.left = left;
});

If I just replace box.style.left = left; with jQuery it works fine

$('#wrap').css ({   
    left: left  
});

but I'm just so confused... Why is box.style.left = left; not working when it's practically the same thing?