The reason I use them anyway is that because they are P.O.D., I don't need to pay them upfront in order for them to make the shirts. They simply charge the amount listed, then produce the shirt and send me a portion. Also, it allows me to experiment with designs and/or shirt styles that may end up being less popular without any risk that I will end up with a huge financial loss (not to mention a warehouse full of unwanted stuff).
By way of contrast, we sell bulk printed tees at Ookoodook, which you will see are exactly in the $15-20 range...but you'll also notice that there are only a handful of designs and each is available on exactly one style of t-shirt. It's also hard to predict what sizes to stock; some of those shirts have sold out of one size while having dozens of another left. As much as I would like to be able to lower the prices, unfortunately it's not really viable to switch to bulk printing for all of my t-shirt needs.