system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Some desktop programs have such an easy time being cross-platform between all the OS's (e.g. Chrome, Firefox, Spotify, Calibre) while other programs basically need to be rewritten from scratch to work on a different OS. What is the reason not all third-party projects use cross-platform code?
gpt
Your premise is wrong. These software have a pretty significant amount of effort going into being cross-platform.

Planning for cross-platform from the beginning makes it look like a low effort endeavor, but it never is.