Answer: Classical thermodynamics was developed before atoms were fully understood.
There are two approaches to studying the behavior of matter:
1. The macroscopic approach [classical thermodynamics]
2. The microscopic approach [statistical thermodynamics]
In the macroscopic approach, a certain quantity of matter is chosen from the study and not much attention is given to what is happening at molecular levels.
Whereas, in the microscopic approach details are taken into account at the molecular levels such as position, velocity, the energy of each molecule, and the behavior of the whole is given by summing up the behavior of each molecule.
The macroscopic approach is only concerned with the effects of the actions of many molecules and these effects can be perceived by humans senses.
All the results of classical or macroscopic thermodynamics can be derived from a microscopic or statistical study of matter.