DriveThruRPG basically subcontracts Print On Demand stuff to LightningSource. I don't know all the details of that, but when a customer puts in an order, LightningSource prints and ships and individual copy for them, so there's no warehousing per se.
But as others have said, being able to set an inventory on itch and ship out books myself--like I do with my zines on Etsy--would be nice too. One of the big things with DTRPG is that I can let people order a POD book and get an electronic version for free at the same time (or set a discounted price to get both, but I just make the PDF free with a POD purchase), and it would be nice to have an automated way to offer that kind of combo through itch.