Hi Reb_enfilade,
I can’t speak for Serta of course but in general it’s because they cost less (which improves margins) and because they can since consumers in general (and most retailers for that matter) don’t know any better and have little idea about the quality of the materials in the mattresses they are either buying or selling.
It’s also not just Serta because most of the major brands also use lower quality/density materials in their mattresses. You can read more about this in many posts throughout the forum such as post #12 here and post #404 here and post #3 here.
It’s not a “mistake” as much as by design. They are well aware that these materials are less durable. A mattress that doesn’t last as long will need to replaced more quickly which will also increase sales and profits.
Phoenix