Earlier quoted context omitted.
Because native GUI on Windows and OSX is better for those users and already exists?
I think native UIs are overrated and just cause headaches for developers. E.g. take the most popular UI system: HTML/CSS/JS: users have no problem with the fact that all their favourite websites look different. IMHO it is a better idea when the application enforces the same UI across platforms (like QT) than the platform expecting the same UI for all programs. On top of that QT does support native look&feel.
A lot of productivity software uses Qt. If it were not using native look and feel it would piss everyone and their grams off when system level keyboard shortcuts didn't work, when menu bars weren't oriented system native, when it didn't use the system file dialog, etc. Integrating with the OS is essential to workflow unless the application literally is your entire workflow.