Logistically, much easier to keep them on the plane. Deplaning would cause far more issues than it solves. The lower doors are virtually the same as most other commercial aircraft, so there are stair options, just not efficient ones. As shown, one set of stairs to offload...
Half of the A380 flights I have taken into Dubai deplane using stairs. It just takes a looooong time.
I am certain Customs is also a consideration.