Honestly it's a very good question. I'm sure it involves a complex interplay of kinetics and thermochemistry.
Why do chemical systems themselves reach equilibrium? Why is it that when you drop a ball it falls to Earth? Isn't that an equilibrium process?
It's the same principle, just with different participants. The dynamics of a chemical reaction are buried in classical dynamics. Remember the collision theory of a reaction.
Reactions are the result of species colliding with one another at an appropriate energy and orientation and all that other good stuff...if these criteria are not met, the species rebound and energy is conserved, ignoring all those nasty dissipative forces.
Throw a bunch of high-velocity particles, molecules, ions, what have you, in a large container, and let them go haywire, and what happens? Some react; some rebound; some react in more than one way, and this is where you have a forward and reverse reaction.
If two molecules react to form another molecule, the product molecule can very well break down to the starting molecules again, because the chemical system is dynamic in more than one way. What's to say that when I get the product molecule there isn't any starting molecules left over? And what's to say that there isn't any energy available for the product molecule to break back down to the starting molecules?
It's very complex, and as a testament it's even complicated explaining such things. Very good theoretical question to ask. Don't hesitate to bother someone with more of an understanding of this idea.