There is a reason! Most smartphones are not really designed to last many years; as such, many will start seeing issues after 2 years or so. On the software side

There is a reason! Most smartphones are not really designed to last many years; as such, many will start seeing issues after 2 years or so. On the software side, carriers are awful at updating Android security beyond a couple of years, for example. On the hardware side, for example, batteries lose capacity pretty over time and it ends up not being worth it to get a new battery. (other parts, too, don't last long enough)