One reason water is easy to circulate through hydronic systems is that it’s incompressible. Unlike air, and other gasses, it’s practically impossible to squeeze a given quantity of water into a smaller volume.
A scientist would probably argue over the word practically in the previous sentence. They would correctly point out that water can indeed be slightly reduced in volume under several thousands of psi. But us “purveyors of the pipe” know that long before our systems ever reach such pressures, their relief valves will have opened, or the buildings they serve will have been launched into the next county.