To keep track with the ever changing APIs, and look&feel, of the host OS.

I agree that some apps are feature-complete enough to be both useful and left without updates for years, but sadly they will eventually bitrot after some time and be increasingly annoying to use. This is usually the moment the maintainer is nowhere to be found as they lost interest, are busy, or dead.

The class of software that never needs updates is shrinking rapidly — unless you're ok with using software that looks like it was made for Windows 98. It's fine, but not great