> If you write logistics software, the company will say "we own all logistics software you write". You can't create a competitor. But if you work for a logistics software company and decide to go write a video editor on your own time, the company wouldn't own that.

The problem is there's no clear legal definition of what "logistics software" is. A video editor is a seemingly obvious example of what is not, but what if you came up with a novel optimization technique which is not necessarily only applicable to logistics? Could you spin off that software into a separate business? What about something more fundamental, like tooling? Think about something like Slack, which was just meant to be an internal messaging tool created as part of the development of a video game. Imagine after Slack took off, that an employee claimed that because they had written Slack at least partly outside of working hours and it was not "video game software", the company didn't have ownership of the software.

This is why the common approach is that the company owns everything except anything explicitly carved out, it avoids ambiguities like this.