Why is density constant?

Hint: Density is found by the ratio between mass and volume. So with this relation, we can answer the given question. The density of a substance changes with the change in mass and volume of the same substance.

Complete answer:
In the case of liquids and solids as there is not much change in their mass or volume the density is said to be constant.
Under most normal conditions, liquids and solids expand slightly with increasing temperature and compress slightly with increasing pressure, thus their densities are nearly constant.
A strong force of attraction exists between molecules in a liquid, which binds them together in a short space. Because the interaction is temperature dependent, it varies with temperature. Because pressure is a fluid property, it has no effect. The size of the volume change is relatively small.
The mass of a material is determined by the amount of matter it contains, whereas the volume is determined by the amount of space it occupies.
For gases, The ideal gas equation is
 $ PV = nRT $
Where $ P $ is the pressure, $ V $ is the volume, $ n $ is the amount of substance, $ R $ is the gas constant, $ T $ is the temperature.
Arranging the above equation we can see that density is proportional to pressure and inversely proportional to temperature.
 $ \dfrac{n}{V} = \dfrac{P}{{RT}} $ .

Note:
The SI unit of density is $ \dfrac{{kg}}{{{m^3}}} $ . The density of fluid also changes, it does so far less than that of gas. As In gases, the interaction force (Van Der waals force) to bind them in a short space is much lower than in liquids. Because it is a weak force, it is easily influenced by changes in temperature or pressure. A molecule obtains energy to overcome the attractive attraction and occupy more space with a slight increase in temperature or drop in pressure, and vice versa.