It doesn't have to, you dingus.

"Rocket thrust results from the high speed ejection of material and does not require any medium to "push against". Conservation of momentum dictates that if material is ejected backward, the forward momentum of the remaining rocket must increase since an isolated system cannot change its net momentum"

[link to hyperphysics.phy-astr.gsu.edu]

It's not a gas diffusing in a vacuum, dummy. It's forcefully ejected matter. Just like a bullet, just like a man throwing a bowling ball. It cannot be made any simpler.