Google just gets too full of themselves. People like to actually read the text in a URL. If you hover over a link that goes to /topic.php?t=13345 it's not very identifiable. However if you hover over a link and it says. /why-google-is-full-of-shit.php you get a better idea of where you are going.
it's also easier to navigate the hierarchy of a site when the categories are things like /porsche/ /bmw/ ect..ect rather than /112/ /123/
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