system
Given the following post, respond as if you're commenting on Reddit:
human
[General] Why do Sci-Fi Navy's use such old ships

I always found it weird that in franchises like Star Trek, Star Wars and Halo the Navy's keep using ship designs and spaceframes that are decades if not centuries old. eg in Star Trek TNG they are still using ships that were old in TOS like the Miranda Class and Consitution Class (Constitution was over 20 years old in TOS).
gpt
Is 20 years old supposed to be a long time for a ship? USS Nimitz is 46 years old and still in active service. Why would the Navy spend time and resources replacing perfectly good ships after only a few years of service?