THX certification isn't required. It is simply a process that a company may put their product through to earn a particular distinction above other products. Some companies, like Marantz, are able to rely on their own brand name for this distinction, because they are known for putting out quality products.

Some audiophile companies have products so far and above the THX standards that it's not worth their time and money to submit their products, and make no mistake, it is very expensive and takes time. This is why companies like Yamaha only submit their top-end products, to make a distinction within their own line of products.