A chair, or static system, has a focal density that stabilizes the mass.
"Air" or in the case of flight, a dynamic system, does the same thing, exactly. Newton applies.
The fact that the "Air has no shape" is irrelevant. Besides, prove that the fluid is shapeless!! There is no mystery. The aircraft creates its own support (by the combustion of fuel) in an infinitely sequential manner.
To go to the extreme, one can say without fear that the chair is not fixed, it is moving!
airbear