If you look closely at those 16” and 20” mattresses in the big-box stores, you will see that they are only made of 1, 2, or 3 layers of cheap and unsupportive foam. Most of the time you will see that the only difference is that they made the base foam thicker. It’s one of the cheaper layers which is why you see others offering a 10” and 12” base foam with a 1.5” top.  Good luck finding sheets to fit those ultra-thick nonsense mattresses.
We offer a high-quality minimal-thickness base layer because we needed as much room as possible to fit all our premium layers above.  Come feel the difference of what real premium layers can do for you.