Rather what is considered good UI and ease of use is sub-optimal for people who understand tech, and those are the people who write free software for themselves and others like them. (Why would you spend months making some UI that in the end requires a minute of manual labor to do what some piece of syntax in a config file achieves in seconds, and which can be diffed to the previous version and so on.)

There are good examples of UI in free software. There are also terrible examples of UI in proprietary software. A lot of the peculiar user experience in GNU/Linux is that way because it was cloned quite exactly from a proprietary system called Unix. A lot of that bad user experience is standardized with an IEEE standard called POSIX, which isn't free.