No offense, but it also depends on how you open the window.
I use the Windows key shortcut to open explorer rather than clicking on shortcuts or right clicking on a folder and select explore, so I get a full screen window all the time. All it took for me was to use the Win + E shortcut, the Explorer window opened, and then I maximized it and closed it . Now that Explorer windows opens full screen on my main monitor.
To be honest, with as many people as are registered here, I would say it is a safe bet that not everyone wants the explorer window to open full screen / maximized all the time - there are plenty of times that I want to be able to move it to the side and use it to find a path to an object, for example, so that I can paste it into a forum post. Other examples like this can ensue as well.
Here is another test - I clicked Start, then typed
and it opened a smaller window. I maximized it and closed it, Then I did the same - and it opened in a full screen / maximized window. Now, this got me curious.
So, I minimized everything on my desktop, and then proceeded to click on My Computer, My User Folder, Control Panel, Network and Recycle Bin (all of which I have set to show on my desktop). The last three didn't open full screen - so I maximized each of them and closed all three of them.
Guess what? All three next opened as full screen.
Now, I have not rebooted (yet) and don't know if they are going to stick - but after I get done answering other posts I'll be happy to reboot and check to see if the settings stuck.