Operating systems are a bit like the computers used to run them. Years ago when Ubuntu was new your computer would be out of date in two years and an upgrade of the hardware was needed. Nowadays it is not like that and machines older than five years are perfectly fine to use on a daily basis. So it is good that your operating system doesn't expire with no further updates given half way through the machine's life.

This is even more so in server-land than desktop, a functional web server that has worked for years shouldn't need upgrading to a new virtual machine so often. Two to three years can fly by in ways that didn't happen when hardware had to be constantly upgraded.