Because you are double clicking in the synth rack not the plugin browser. To insert a NEW synth double click its name in the plugin browser tab.
BTW there is an exhaustive shortcuts list for the browser listed in the
new fine print which is linked at the top of the forum. There is even a gesture to drag and replace the contents of an existing effects bin by holding down the ALT key when dragging. Not exactly replacing a single plugin but its a step in the right direction. Full synth replace is something we definitely have planned. Its easy we just don't have enough hours in the day to get to all these details. Now when is somone going to write a real time expansion plugin? :)