I have maybe 10 shirts in work-rotation, and a few others that are either leisure shirts or just get worn less often. So, maybe the most-used ones get worn between once a week and once every two weeks. The oldest are maybe four years old and don't look like new, but I have some 3-year-old Brooks shirts that have seen a lot of use and are still going strong.
I think one factor is care and feeding. I go as long (and as many wearings) between washings as I can. To support this program, I wear undershirts to work year-round. Washing (and pressing) really takes a toll, probably more so than wearing in some ways. So if you are just washing a shirt after each wearing, ask yourself if that's really necessary. If you're using a laundry, ask yourself whether they're really taking care of your clothes. Odds are, the answer is no.
That said, high-quality fabrics will make a big difference. I'd be pretty disappointed if, wearing my shirts once a month, they felt worn after 18 months. I mean, that's probably less than 20 uses. Higher-quality fabrics, woven on slow looms with long-staple cotton will last longer than less-good ones. The lower-quality shirtings willl develop invisible breaks in the fibers more quickly, destroying the hand and leaving them feeling limp and worn out.
In short, it sounds to me like your Jantzens are wearing out too fast. But that could be any of the factors listed above, or a combination.