Why are liquids not mostly measured in weight or mass?
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Liquids can, by their basic definition, fit near perfectly into a space — a tube or a jug, for instance. Things like water, milk, blood, hydrogen peroxide, etc. We can and do thus measure them in terms of volume. A milliliter, for example, which is the amount of space occupied by a cubic centimeter (about the size of a playing die). You can still weigh these things. Everything has a weight/mass. But volume is more useful.
Unlike fluids, things like couches, televisions, bodies, rocks, and cars do not fit perfectly into containers. Their weight is also not evenly distributed, unlike a liquid. Measuring these in terms of volume doesn’t tell us as much as it relates to quantity. If you fill a liter jug with rocks, the weight will probably be different if you filled it with pebbles than if you filled it with stones. Only liquids (and gases) make sense to measure by volume because the space they occupy is uniform.
With gases though you have to go the extra step of defining the pressure, which can vary if you confine it to an area.
Edit: It’s true that you can measure a room in terms of cubic meters and thus liters. But that’s not really useful other than knowing how much space you have. My presumption with this question is that you’re asking why we measure some things in terms of volume and not others.
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