That's because the thing that makes them heavy and stiff will make them last longer. You have two options. you go with a boot that has a lot of synthetics in the sole so it is confortable immediately. But those boots will have a shorter life because they aren't repairable and more because the foam and cushioning in the sole break down over time. So an example is running shoes. I have running shoes in my closet that you would think are still usable because they look fine. But the sole material has failed and I don't use them anymore. Running shoes are 300-500 miles before failure and that is true to me. So for me I go through a pair of running shoes yearly regardless of what I pay or what they look like as the sole will fail long before the upper does. Hiking shoes will last longer but they are more like 1000 miles. Also depends on how much you weigh and other factors.

On the flip side a heritage boot doesn't have this synthetics. So they are harder to wear but they will last far longer, and can be repaired. So this just comes down to what you want. Most people are used to modern shoes that support and cushion so have a very hard time going to a heritage type boot. So basically any modern shoe will have a life in miles before failure. Most people wear these types of shoes for far too long and that can cause problems too. If you like that type of boot then buy two pairs and rotate them. But know that if you do get all the synthetics that they will likely fail at the same kind of distance used.