Sam and I discuss a lot of things when we are in "leather mode." Mostly, we talk and either design or time. Design and implementation occur on any leather project. But the concept of time is almost entirely related to the business of leather. The great thing about leather is the market for leather is really extensive. This can also be a downfall, but more on that later. Leather is a "high-end" product that can fetch a price that is much higher than the costs of the parts (depending on design and construction methods). Fortunately, hand-made leather is almost always more durable than machine made products (especially cheaply constructed). The very nature of hand-stitching is more durable than most machine stitches.
First, the thread is typically thicker and synthetic. If you see a leather item with a thin, cotton thread, it will eventually fail. Now, it may only be a "one year bag." In that case, it could last that long. But quality leather ages so wonderfully. Why would anyone buy a one-year leather bag? Or even two year? On full-grain leather that comes from a top tannery, the scratches, nicks, and color variations have more character than your favorite pair of broken-in jeans.
The second reason that the thread may fail is due to the stitch itself -- the loop stitch. We have all had that shirt where we pulled on the wrong stitch and started the unlooping process. Well, hand-stitching typically involves some version of a saddle stitch. This involves double stitching. Even our cross stitching is a modified saddle stitch. And, as the name implies, a saddle stitch is used on leather saddles. Now, if it is the trusted stitch of saddle-makers, I trust the stitch to be more durable than that Chinese knock-off made of "genuine leather."