why is it possible to talk generally about all gases when we cannot do this with solids or liquids?

Answer 1

Because gasses, which have very low densities, have minimal molecular interactions, which cannot be said for solids or liquids. However, when the conditions for gasses are changed such that the interactions can no longer be ignored, the identity of the gas does play a role.

Answer 2

We do indeed talk generally about all liquids and solids as we do about all gases. You need to ask your question again more clearly.

Answer 3

Solids and liquids are incompressible, therefore do not change with direct proportion to temperature and pressure as do gasses.