Why is Hess's law true?
Hess's law is true because it embodies the laws of thermodynamics.
The three laws of thermodynamics hold that, (i) you can't win; (ii) you can't break even either; and (iii) you can't get out of the game. You might be able to find more appropriate definitions. Every observed physical and chemical process is known to follow these laws.
I'm not sure the three principles are really needed to justify Hess's law. I think the property of energy and enthalpy of being state functions is enough.
In other words, the chemical system can exist in the reactant state and in the final products' state.
These two states of the same system (formed by the same kind and number of atoms) correspond to two given and precise ethalpy values or "levels".
Thus the transition of the system from the first to the second state entails a change in enthalpy which is the difference between the products state enthalpy minus the reactant state enthalpy, independently by the number and quality of intermediate steps.
And this is the Hess's law.
It is the same to say that the difference in altitude between Central District and High West is fixed in 490 meters independently if you go there from your home passing through Peak Tower, or via Pok Fu Lan rd through country Park.
Altitude is a "function of state", as enthalpy is a function of state. This is why Hess's law is true.