What can cause a delay of several weeks between an editor receiving reviews and you receiving a response from the editor?
There are a number of possibilities. Off the top of my head:


The editor is traveling or otherwise occupied. In my experience, this is particularly likely if you are dealing with a journal that employs professional editors rather than relying upon academic editors. 
The reviews are mixed and the editor has decided to careful assess the manuscript himself or herself--which may take as long as writing a review would take.
The reviews are mixed and the editor has decided to seek additional reviews, but this is not yet reflected in the manuscript management system. 
The editor wishes to speak in person to one of the referees to clarify something that the referee wrote, but has not yet been able to reach the referee.


You could go on and on and imagine any number of other possible scenarios. I agree it is not optimal for this to happen, but nor is it unusual. As an editor, I try very hard to return a decision within 2 or 3 days of receiving all reviews, but as an author I have found that waits of a week or two are not unusual.