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Actually, what happens inside the cylinder is not really an explosion. It is a burn, though a rapid one. Explosions are what you hear as detonation or pinging, and those are uncontrolled and can damage the engine. They're not more powerful than the normal operation, they are not controlled burns.
The proximity of the molecules to each other has some effect on the burn, but it does not have any effect on what ratio of reactants produces a complete reaction. That has purely to do with the number of molecules. You also seem to have been assuming a linear relationship between compression ratio and burnable mixture, and that is certainly not the case.
If the mixture is not close to the stoichiometric one, undesirable things happen. If it's too rich, you wind up with unburned fuel going out the exhaust, resulting in high HC emissions. If it's too lean, you wind up with elevated temperatures in the chamber and high NOx emissions. Those are both undesirable, though some of each can be tolerated as a necessary evil. Too far in either direction will also result in a loss of power, which is also generally undesirable.
The really big thing that happens when you stray from the stoich mixture is that standard (relatively inexpensive) catalytic exhaust converters stop working as effectively. So all levels of pollutants will potentially increase, or at the very least the total amounts will increase. Again, this is undesirable.
There are expensive catalysts and more sophisticated engine management strategies that can work better with leaner-running engines, but they cost more and are only moderately effective. They could likely be improved to where they do meet the current emissions regulations, but there isn't really any incentive for the manufacturers to do so. The cost will be significant, and basically the same benefits can be had for less money by using a smaller engine running more or less conventionally paired up with an electric motor.
The whole hybrid thing is very well understood, is being widely produced so the economies of scale are working for it, and has effects that are fairly close to those that would be achieved by investing a lot more $$$ into making a lean-burn setup that does everything that the automakers want it to do.
Lean burn may still be the way to go in the future, but at this point it isn't worth the investment for the manufacturers. Maybe the 2025 standards will force them to move that direction, or maybe not...
-soD
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