Support forums such as this one are a lightning-rod for people with problems. Apple sold 20 million Macs last year, and I do not think you saw anywhere near 20 million complaints of broken headphone plugs. It is very difficult to tell how widespread a problem is by reading support forums. All you can accurately imply is that more than a handful of people are having a problem.
The jack is fully shielded (so there is no access to its "guts") and soldered to the motherboard. Apple does not solder boards in the field, they repair by replacement.
This is not a "defect in materials or workmanship", so Apple does not see huge costs of repairing these under warranty. Perhaps if they did, perhaps there would be better solutions.
I expect they would tell you to use only high-quality headphones (such as the ones they sell) that do not suffer from these problems. Or maybe the headphone jack should be on a peripheral device instead of directly on the expensive motherboard.