When book trying explain on liquid pressure, they always explain by mathematics sense. But what i dont understand is that the book always treat liquid as macroscopically one big system [to find a mass by $\rho × volume$] instead of discrete liquid molecule (example if you dive under water the pressure are calculate by the weight of water above you). If you ask me why not. In solid the particle are stick together but for fluid [liquid] they are different where they can flow.
So my question is, why do they treat liquid in that way in order to find pressure? If the answer is that they won't be any different, then can anyone provide some explanation regarding that?