This happens because some goods are significantly easier to make than others. As a result, you will see more items that are simple to make being sold than those that are rare or complex to make. After all, all items that are listed for sale in the markets are sold and purchased by real life players. Additionally, the newspaper has a limited amount of pages, which means that given the large amount of people playing the game, it is possible that there might be lots of similar goods listed in it. Please note that the newspaper will only have goods that you already have access to based on your level.