It is because computers run one of a few available OS's. The OS is being maintained by the distributer (MS, Apple, Google) and your hardware is good as long as the drivers are still receiving updates.
Phones are different because even though everyone only uses iOS or Android, every Android manufacturer puts their own layer onto Android, so Google can continusously update it but the manufacturer might not. Most companies only maintain their phones for about 3 years, giving a significantly reduced lifetime than computers.
It still works fine, from from a security perspective, keeping the phone without patch support is a bad idea.
