Hello Frans
Autodesk is not releasing the extensions or add-ins directly with the software, as the software itself needs to be developed, and the extensions work on top of the finished software.
No add-ins can be created until the design of Revit 2018 is complete. The reason for which these complements are released later is that Revit, on its own, can be used to work just fine.
The design of add-ons, plugins and complements to Revit takes time, as each has to be modified from the previous design to fit into the newer version of the product. Also, with the software just being recently released, it is unthinkable to consider all releases of plugins will be done in tandem.
These are created and released periodically, so that they can be implemented and tested incrementally, and avoid the opportunity of these plugins causing issues with each other, due to a hasty release. The programming needs to be done carefully, so that all of these functions in Revit can be used adequately, and reduce or eliminate entirely the possibility of internal conflict due to this programming.
I do hope you understand that this is not by any means an action to hinder the progress of our customers, but rather a delicate schedule to keep balanced the functions of the existing software with the implemented changes for the newer version.
Dan Faba
Autodesk Revit Technical Support