If you know anything about the durability of liquids, you know that they're nearly impossible to compress.
You can pour a gallon of milk into a tall and skinny gallon container or into a short and squat gallon container.
However, you'll never be able to get that gallon into a half-gallon container.
It's not as if it's completely impossible to compress liquids, but it would take massive amounts of pressure to make any noticeable difference in volume.
And under those kinds of circumstances, most nearby elements would be struggling with the compression.
Gases
Gases are easier to compress than liquid because there's much more space between the molecules.
If you neglect the amount of compression that a gas is experiencing, you could make huge errors in calculation.
Liquid flowing through a pipe at full volume takes the shape of a pipe, but a gas would not.
The behavior of liquids is easy to predict, as it must maintain the shape of the pipe it's flowing through but the behavior of gases is a little more challenging.
Summary
Liquids are nearly impossible to compress, which makes their behaviour very predictable.
When you're calculating flow rate, gases are going to be more complex than liquids due to the amount of space between molecules.