Here's why:

* Kinetic energy is the energy of motion. In a substance, this motion comes from the individual molecules vibrating, rotating, and moving around.

* Thermal energy is the total kinetic energy of all the molecules in a substance. It's directly related to the temperature of the substance – the higher the temperature, the more kinetic energy the molecules have.

* Internal energy is a broader term that includes both the kinetic energy of molecules and the potential energy associated with the forces between them. However, in many cases, the kinetic energy component dominates, especially at higher temperatures.

So, while "kinetic energy" describes the energy of individual molecules, "thermal energy" or "internal energy" refers to the total kinetic energy within a substance.