Qt, on the other hand, is its own universe. It's written in a weird dialect of C++98 (though I'm sure it works just fine in C++11 these days), it has its own object model, networking stack, container library, threading library, graphics primitive library (i.e. not Cairo). This object model also leaks into its language bindings if you don't want to write your software in C++.
It's the same problem that Java and C# also suffer from: they're not cross-platform, nothing is. What they actually are is their own platform built alongside a perfectly good already-existing one, and you can see the seams.
There's more to each platform's UI than what bitmap you skin buttons and checkboxes with. If you want a cross-platform application, then write a completely different UI for each platform using those platforms' native UI toolkits. Sadly "good enough" is the order of the day here, so you end up with platform-refugee applications that look like shit.