The YaBBC codes that the forum uses are much simpler than their HTML counterparts. Table code in YaBBC doesn't take attributes. Things like table/cell width and alignment have to be done on the sly with other YaBBC codes or blank images. And things like boder size/color aren't available at all.
The truth is, I've thought about adding attributes to the three table tags so that they would be more like HTML, but I don't believe that anyone other than myself and Midnite has ever even used a table in a post, and for what we've used them for, attributes weren't really needed.