Unfortunately, it's just a fact that some reactions are reversible and others not. Burning fuels is not reversible, but the Haber Process is. Reversible reactions tend to be more complicated and they usually proceed via an intermediate or 'transition' state. The Haber Process is a case in point. There is a lot of complicated chemistry going on involving binding of these transition complexes to the iron catalyst. At any point in the foward reaction, one of these transition states might break down, and instead of proceeding towards products, will fall back into reactants. Don't forget that reaction kinetics is also closely linked to dynamic equilibrium. The latter is reached when the rates of the forward and backward reactions are equal. Rate is also affected by the transition state.
However, it's not quite correct to say that you can't harvest products at equilibrium. Sometimes it's useful. In the Haber Process for example, you can condense the ammonia and remove it. The equilibrium will then move to the right to make more ammonia which is what we want. Adding more reactants will have the same effect. The fact that this reaction is reversible is useful - we can just keep adding reactants and removing product to keep the reaction going. It's also possible to fine tune the process by altering reaction conditions.