Hi mtjag,
That’s because Simmons and the other major brands tend to use lower quality materials in the comfort layers of their mattresses that tend to soften and break down prematurely which results in the loss of comfort and support which was the reason you purchased it in the first place. This is why I would avoid the major brands or any mattress where you can’t confirm the quality and durability of the materials in the mattress and in the upper comfort layers especially (see the guidelines here). The issue of lower quality materials is more common in the industry today than it was 17 years ago (see post #3 here)
This is just marketing and is very misleading. You can read more about so called “plant based foams” (which aren’t natural or plant based at all) in post #2 here. It’s just polyurethane foam but at least it’s good quality/density polyfoam.
Phoenix