I'm nastily allergic to wool, but as a knitter and spinner I know a fair bit about the stuff. Sock yarns that are available tend to be 80% wool and 20% synthetic. This is essential to prevent wear at the toe and heel- if using a 100% wool yarn we often use a stronger (more abrasion-resistant) holding yarn along with the wool at the toe and heel. Wool makes elastic, springy socks that are very tolerant to fit issues but it can be worn down very easily. This is why historically stockings were made with replaceable toes and heels in a different color to aid picking up the stitches again.
A huge amount of the durability you see in yarns depends on how they're spun. Fibers are strongest when plied, though if the plies are any thicker than hair-thin this can make a thicker garment. This isn't so much an issue in socks (which always use as many plies as possible) but is an issue in shirts. Generally, the more plies and the more twist a yarn has in it the more durable it is. The downside of this is that it feels much stiffer and harder. Softness and fluffiness are qualities that move a ball of yarn off the shelves but that generally make garments that shred very quickly when worn. Fibers come loose from the yarn very easily when there isn't much twist to hold them in.
When you're giving a shirt or sweater the rub test in the store, think less about softness and more about checking the rubbed area for any fiber ends that spring loose. Merino is one of the least durable varieties of wool, given that it's bred for extremely fine fibers along with the fact that Merino wool has very short staple length so there are a lot more ends to pop out and come loose. Merino is used because there's a glut of it available cheaply from Australia and because you can wear it next to the skin.
Better breed choices for making high-performance wool clothing are down breeds like Dorset and Shetland. Down sheep have fiber with a spiral crimp that produces ridiculously high loft, springiness, and warmth. Down wool also is nearly impossible to felt, which can be ridiculously easy in soft Merino unless you chemically strip the fibers to make superwash wool. If anyone out there wants to start a push to use down breeds for technical clothing, knitters will back you up. Merino is a scam.