- Concentration of hypochlorite ion in bleach

Let's see. Basically you are on the right track. For the why question, I don't really get your problem, because you can easily say, the hypochlorite-ion is very unstable, which is true. Cl and O does not like each other very much, they have very similar electron negativity, in other words they have the same "strength". Would you pick on a man with the same strength as your or would you tease a weaker one? Of course a weaker one, not very nice, but atoms like to do this as well.

So with hidrogen being the weaker kid, they easily suck its electron, that is why HCl and water is quite stable. You force together O and Cl and they have a hard time sucking electrons from the other because strength is almost equal. Besides they have more than one lone electron pairs, and these take up a huge space. So they push each other away, because of their electrons as well. hypochlorite-ion is not a good ion for oxygen nor chlorine. So it can easily break apart. Forming a Cl- and an atomic oxygen, and this is the power of bleach. hypochlorites are used in bleaches, because when they break apart they release this atom of oxyen. not molecule, atom. And this has high reactivity. It destroys the structures of other molecules, by bonding and breaking the material. Bleach is therefore great to clean dirt out of your T-shirt, but if it has color, beware because oxygen will destroy the molecules of that color too. This is why bleach can only be used on white material.

Anyway there are new detergents nowdays, that are based on organical chemistry, and you can wash every kind of cloth with that.

How you make hypochlorite?

Cl2 + H2O <-> HCl + HClO is the correct equation between chlorine and water, but with this you can not make use of hypochlorite.

Cl2 + 2 NaOH ---> NaCl + NaClO + H2O this is what the they usually do, and the material in bleach is sodium-hypochlorate, in water releases hypochlorate-ions which are again very unstable, so they break into chloride ion, which again with the sodium ion forms salt, and the other one is atomic oxygen. Which again is great for destroying compounds.

So the answer to your question is quite simple, let's state that you have a hypochlorate acid solution.. because you can't just form hypochlorate-ions at start. So this hypochlorate acid breaks up to H+ ions and ClO- ions
HClO <-> H+ + ClO-
ClO- again brakes up to atomic oxygen and chloride-ions atomic oxygen in air exposure flows out from the solution, chloride-ions remain in the solution. So with time because of the instability of the hypochlorate-ions atomic oxygen flows out. In hypochlorate-ion chlorine has +1 oxidation number!!! Although it is extremely rare for a halogen to form compounds with other than -1...!

ClO- --> Cl- + "O"

Your equation has some problems.
O2 + Cl2 <---> 2OCl
This can't happen, because for one thing you left out the charge of the hypchlorate-ion... and if you leave it, you would probably see that this is not a equation because on the two sides of it there are different charges.

O2 + Cl2 <--> 2 OCl- is a bad equation... from two minus charges becomes zero. And hypochlorate breaking apart is not an equilibrium.

HClO --> H+ + Cl- + "O"

The why is not this equation, the why is that it is very unstable, chlorine has +1 oxidation number, although it must have -1 to achieve the noble gas state... So it is very likely to break apart.