It's because stackoverflow, like many sites, uses a widely accepted 960px page container width standard.

Within your full-width footer container, put a second, invisible container for the content, whose width is the same as the width of the main document body, e.g. 960px. Here is stackoverflow's body container:

#content {
    margin: 0 auto;
    width: 960px;
    min-height: 450px;
}

And here is the container for the footer content:

.footerwrap {
    margin: 0 auto;
    text-align: left;
    width: 960px;
}

Additionally, I don't think the footer 'sticks' to anything. It's simply at the end of the document, so it's rendered at the bottom of the page. I might be wrong as I haven't looked at stackoverflow's html source in that much detail, but anything else seems like overkill.