Why gases spread quickly?

They have a very low viscosity, and expand to fit their container.

Gasses have a higher energy per particle than solids or liquids, and so when they are in a room with lots of space the particles move to occupy all of the space.
This is called Diffusion or Brownian motion.

Again frany, please check the date of questions you answer, this one is several years old.

Dueto more brownian motion
Due to less air molecular disturbance as compared to offered in water and solids.