They don't. What you are probably referring to is how they reimplemented lots of components from kde3 to kde4. This was due to a switch to a new major qt versio

They don't. What you are probably referring to is how they reimplemented lots of components from kde3 to kde4. This was due to a switch to a new major qt version, at which point porting is a big enough amount of work that you might as well improve things while you are at it.