Because it’s a playing technique, not an articulation.
But I agree with you, it’s in the wrong place by default 
There isn’t even a way to move it, unless you wanted to move every single one by hand in engrave mode.
Yes there is!
You’ll need to implement in the part separately (or propagate properties for the 8va line from within Engrave mode).
Cheeky! Great stuff as always, Leo.
One old trick from Sibelius days one can carry over to Dorico is changing, for example, the stress articulations into the harmonic circle, so that it behaves.
Dear Leo, thanks for the tip!
I notice that you need to click twice on the override button in the properties panel, that is a problem I experience quite often since Dorico 3, I hope this little glitch will be solved, it’s a little bit disruptive in the workflow.