Sometimes eye candy actually helps usability. There was a time when I would have argued Apple did this best, but lately I feel less this way. Still, look at apps like Zed. Beauty in its simplicity. That's worth a lot.
What frameworks like Qt often suffer from is an outdated or overly restrictive design language which is harder than worthwhile to customize, so you get apps that feel antiquated. Some people aren't as bothered by this as others, and generally I agree it shouldn't matter, but it does.
"Eye candy" also helps prevent the uncanny valley effect, but for ux.
On Mac OS, I can almost immediately tell when something is a QT app. It can be a beautifully well designed app, and my hindbrain is just screaming that it's not right. Take QGIS. Looks good, works well. But doesn't feel like a Mac app.
And it's not even about following all the Mac conventions. Sure, I like the Mac style preference panes OmniGraffle and Transmit have. But the telegram Mac client, written in swift, has it's own weird preference system, and still feels "right"
Electron apps, ironically, don't trigger the uncanny valley effect. Maybe it's because they all bring their own UX, instead of trying to ape platform ux, maybe it's because the font and other rendering is more polished, by virtue of being browser descended, but either way, they don't make me feel like I'm interacting with some weird not-quite Mac program.
Not all software is used hours a day. Some software needs to be intuitive so the user doesn’t need to invest hours to learn it. And visuals play a huge part in that.
What frameworks like Qt often suffer from is an outdated or overly restrictive design language which is harder than worthwhile to customize, so you get apps that feel antiquated. Some people aren't as bothered by this as others, and generally I agree it shouldn't matter, but it does.