Why swapping multiple colors of multiple objects at once not possible?

InDesign can do it by default. In Illustrator you may use a script.

Among others, @femkeblanco has written a good one (see below).

// Script by femkeblanco
// to swap fills and strokes of selected items
var array = [];
function push(items) {
    for (var i = 0; i < items.length; i++) {
        if (items[i].typename == "PathItem") {
            array.push(items[i]);
        } else if (items[i].typename == "GroupItem") {
            push(items[i].pageItems);
        } else if (items[i].typename == "CompoundPathItem") {
            push(items[i].pathItems);
        }
    }
}
push(app.selection);
for (var i = 0; i < array.length; i++) {
    var fill = array[i].fillColor;
    var stroke = array[i].strokeColor;
    array[i].fillColor = stroke;
    if (!array[i].stroked) array[i].stroked = true;
    array[i].strokeColor = fill;
}