Typically, manufacturers make parts in batches, and the sill plates you are wanting might not have enough demand to be manufactured constantly, so they are made when there are enough orders to warrant production setup.
By placing your order, that could actually shorten the wait time because there are now additional parts in the mix.
I suspect that there might be just one manufacturer for your door sills and that they are waiting until they have enough orders to manufacture them. Or, CARS might be a reseller for these door sills, and they may have to place a minimum order to get a good price on them. It could also be a combination of both.
You may also want to try Trim Parts in Lebanon Ohio. They manufacture many of their parts, and have a good selection of sill plates to choose from. Many times the "Fisher Body" sill plates fit brands other than those they are made for, such as Chevy may fir Pontiac or Buick. It's worth a try to contact them and see what they have.
I recommend that if you really want these parts, you should place the order right away. In the meantime, if you find them somewhere else, you can always cancel your order with CARS.