This was a deliberate decision in PageFour and also in Atomic Scribbler. My feeling on this is that the main working part of the UI — the word processor — should not have its content changed by a single action. Clicking in any of the trees is a very casual action that often happens not because you want to open and work or read a scene, but simply because you’re navigating the tree.
Causing the content of the working word processor to change based on a single click action would — to my mind at least — get very frustrating for a lot of users very quickly. Most of the time spent in an Atomic Scribbler project will be spent writing inside one scene at a time. I’m extremely reluctant to have a casual click elsewhere change the active scene in the word processor.
On top of this, there are extra elements in the Research tree apart from scenes and notes — images, files, etc. Using a single click to open, and being consistent, would cause external programs such as Adobe PDF, image viewers etc. to launch and come to the forefront after each single click — again, something that would be immensely frustration if all you’re doing is browsing through the Research tree.
I might look at allowing this behaviour as an option down the road, but personally I think the cons outweigh the pros here.