Content production tools have fairly unique UI needs compared to the average desktop application, e.g. they often have very complex timeline controls that offer dozens if not hundreds of different interactions specific to the tool at hand. Generic UI libraries would only be a hindrance there. They almost never have enough screen space, so the widget/interaction density is far greater than what’d be ergonomic for standard apps. Since people spend considerable time learning these tools they generally have their own UI paradigms independent of the host OS, which is the opposite of what you‘d want in a crossplatform toolkit.
I have argued in the past that any app worth a damn will not use some generic off-the-shelf widget kit. This includes even apps such as Excel or Word.
The reasons for this are that GUI widget kits are generalized to the least common denominator. They are written with a specific, narrow usage in mind and will never fit your application to the degree you need. And in some cases the author of a certain widget tries to do this, and the widget succumbs to the weight of complexity required to be all things to all people. Not even GIMP uses only GTK+.
The second reason that staying in one widget kit is bad is because it's a competitive world out there. You need to differentiate. That's not easy when every app looks and feels the same because they all use the same tool kit.
In addition to all of this, different OSes have different UX. A Qt app on Mac is not going to be identical to a Qt app on Linux. Assuming the Qt app is written to the guidelines of the OS and not just doing whatever it wants. Internally, the app will do what is best (e.g. photoshop canvas), but will play nice with the external window manager, accessibility, etc.
Word/Office is such a good example because they use their own window decorations, ribbon, scroll bars etc., which no other program (even from MS) has, and no second source implementation with exactly the same features exists.
the question is not really whether they use Qt. It's how much of Qt they use. Ardour, for example, uses GTK, but hardly uses any of GTK for the majority of the GUI.
