Dixie Gunsmithing,
It's certainly possible to build a spring that will last nearly forever, even if left compressed. However, there are usually other considerations that prevent the designer from considering ONLY spring life, or any other single design parameter, for that matter.
For example, in a magazine spring, the spring tension can't be so high that it deforms the magazine feed lips or body, squashes the rounds, is so difficult to compress that the magazine can't be fully/easily loaded, or that it creates so much friction between the top round and the stripper rail on the slide that it prevents the slide from moving freely.
In addition, the spring needs to take up as little space as possible to maximize the magazine capacity for a given size.
And, of course, magazines can't be too expensive or buyers will complain, and that means that the components of the magazine can't be too expensive either.
All of those considerations are, at the top level, in direct opposition to the design goal of maximizing spring life. In other words, the magazine designer must balance all of his design goals, many of which conflict with other important design goals to find a workable design.
The magazine he ends up with will probably have somewhat lower spring life than he would have liked, be slightly harder to load than the shooters would have liked, hold a round or two less than the marketing department would have liked, cost a little more than manufacturing and sales would have liked, etc...