Conservation is the cognitive understanding of whether or not the amount of a given (inert) solid or liquid will change despite its presentation. For example, human toddlers don't grasp conservation, and will erroneously believe that pouring water from a tall glass into a wide bowl makes more water, since it "looks" bigger in the wider container (e.g. it takes up more visual space in a wider vessel, and therefore must be "more"). No other animals possess this skill either (thank you Wikipedia [0]).
Combine that with the nebulous nature of gas - it's not intuitive to understand that an empty glass is, in fact, full of air - and understanding phase changes become that much more difficult.
It's kind of like how most people won't intuitively understand how discrete phases can break down at higher temperatures and pressures, or the differences between different lattice configurations of the same substance. If it's not something we encounter every day, or aren't taught at an early age, the intuition simply won't be there.