Another problem here is that unix DEs in general (and this includes even proprietary ones like CDE and 4Dwm) tended to solve problem that was cool in the mid 90's but then stopped to be relevant: having consistent cross-application object-oriented model for documents and preferably everything the user wants to manipulate. When you have multiple competing frameworks to implement this (and each KDE and Gnome went through at least one backward-incompatible reimplementation of this) you end up with applications that work only as part of the same framework. The whole OLE/COM/ActiveX on Windows is mechanism meant for exactly this and is used almost everywhere (Explorer window is COM component that displays COM components and the whole thing has exactly zero concept of files and directories).

This whole mechanism is in fact one great waste of time because the probably only meaningful user-facing application for this is embedding Excel charts in Word documents and in that case it is too generic and has some surprising behaviors that confuse users (I assume that what happens when you open document that has embedded OLE object for which there is no installed application has at least something to do with why Microsoft started selling Office instead of separate components) and can perfectly be implemented completely internally by the office suite, LibreOffice has internal system for this and nobody seems to care. One would assume that the useful use-case is more general, like "embedding picture-like things into Word documents", but I've never seen Word document that is actually useful and embeds something other than Excel chart or maybe worksheet, particularly I've never seen Visio diagram in Word document as an OLE object instead of image.

Currently KDE has it's KParts that are actually used in the implementation of KDE and KOffice, but Gnome seems to have completely dropped any such idea (which probably makes sense because their implementation that involved CORBA and could at least in theory work between X clients running on different physical computers was horribly over-engineered and probably was never used for anything that both worked reliably and was actually useful for end user).