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Why can a substance change gradually from a dense fluid to a gas above its critical point?

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argon phase diagram — liquid Figure 1: Phase diagram of argon.

A substance can change gradually from a dense fluid to a gas above its critical point because the distinction between the liquid and gaseous phases disappears.12

At the critical point, the densities of the liquid and vapour phases become identical, eliminating the boundary that normally separates them. Below this point, crossing the vapour-pressure curve results in an abrupt phase change with distinct shifts in physical properties. However, above the critical point, the substance exists as a single fluid state.31

By following a path that avoids crossing any phase-change lines, a substance can transition from a dense fluid to a gas through a smooth, continuous change in properties. In this state, often called a supercritical fluid, the substance maintains densities closer to those of a liquid while possessing the ability to flow like a gas. Consequently, there is no clearly defined moment of conversion between the two states.142