Because the option is named very, very poorly.

As is pointed out here, the name of the option you are looking for that will change the window behavoir back to how it used to be in previouse versions is hidden under Edit -> Preferences -> Window Management -> “Window Manager Hints”.  Yeah that’s right.  Poorly named to anyone but the developer that made it.  Under that change the Hint for toolbox/other docks options to “Normal Window” instead of “Utility Window”.

Restart and there you go!

I got a better idea: Use your feet.

I originally built my foot interface for MIDI/gaming applications but I found another great application: image editing.

I was going about filling around some lines with colors in The GIMP, and as I was working I thought, Hey, I keep switching back and forth between the paint tool and the eraser tool pretty often. When I had to switch I either needed to skim through the giant tool pallet to find my new tool, or laboriously, lift my head and chin from my left hand so I could press the appropriate keyboard shortcut.  It got old, but then I thought: Hey, I’ll just use my feet.

So I fired up my AutoHotkey editing skillz and bound a couple of buttons on my foot controller to the paint and erase tools (or rather the keyboard shortcuts for them).  Viola!  Instant speedup.

Now I can waste away my life painting away at pointless things much more quickly!

Also, I bound another couple buttons to undo/redo to help speed up fixing my mistakes.  Now I just need to figure out how to make the tools pressure sensitive using the foot pedals . . .

Edit: I also set Page Up/Page Down to buttons.  Imagine how convenient it is for me to just lean back and read something long, hands free!

Bewarned: I don’t speak for all the art of all these people, some of it derserves to be skipped.