There are dozens, if not hundreds, of companies that make add-ons for cars and trucks. Big companies don't go after the pennies, they go after the dimes and dollars.
Besides, do you really understand how many supplements are out there? It's one thing for a small publisher to take a risk on a product that may or may not have a good return on investment. If they fail, they're out a relatively small amount. If WOTC puts out too much product (even quality stuff) at best they dilute the brand, at worst they kill the cash cow. It's just not worth the risk.