I would say there are several factors. Qt itself is a very powerful and feature-full framework already offering a lot as a base. So by reusing it and building on this base KDE developers can focus on additional things. And even on top of this KDE developers are very good at creating their own KDE Frameworks providing additional abstractions and common code that all KDE applications can reuse and also provide a lot of consistency/integration between them. Finally KDE is also one of the biggest FOSS projects (I think it is the second, right after Linux kernel) so by having so many people and using the best in class GUI framework is what makes it possible to create so many powerful apps.