That's the life span of these. The thicker ones don't necessarily last longer but it may seem that way because they get used less. So you ask how so? Because they're heavy and clumsy do they get used less.
It makes sense. Inspired by your idea, I’m also thinking of buying a cheaper pool cover.I always buy the cheapest and lightest one I can find. It's only going to last 2 years, and I find the lighter ones retain heat just as well as the heavier ones, but are much easier to roll up and take off.