Why do some kits call for corn or table sugar?

Corn sugar (dextrose) and table sugar (sucrose) are cheap fermentable sugars that produce alcohol without the residual sweetness of malt extract. Their use in quantities of 20-30% will produce a beer with a considerably lighter body than one brewed strictly with malt sugars. Any canned kits, and the body of the beer they produce, can be improved greatly by substituting the corn sugar with malt extract, either dried or liquid.