DVDs are inscribed by a laser indentation of a metallic alloy or dye. The number of rewrites are finite (about 1 000 writes). As a consequence, each progressive indentation is less pronounced than the previous until that point that the DVD can no longer be re-written.
Keep in mind that on rewritable CDs and DVDs, files are are written to the disk in a manner that will maximize its life. That deleted file may be deleted in name - marked for overwrite, but only physically overwritten once the entire disc has had something written on it.
You can read up more about this in Paul Crowley's CD and DVD forensics.