Why hasn't life on Earth reached an evolutionary stable strategy?
Because the environment on earth is not stable on the long run.

Antarctica was on the tropics. However plate tectonics has moved the continent from that location to south pole. And in time the continues will leave the poles and move toward the tropics again.

So a perfectly evolved animal in tropical Antarctica 200 million years ago, would have to continue evolving or it would have gone extinct as Antarctica moved southwards. And the animals living on Antarctica today will have to evolve as the continues continues with its motions.

Also predator-prey relationships... are red queen type relationship. They may appear stable on the outside, but both parties are actually under selection pressure to evolve, to counter the advantages of the other party. They are in an arms races. Pressure to out run the predator. Pressure to outwit the prey. Pressure to better camouflage. Pressure to detect prey from multiple senses (sound, sight smell). 

So on the long term, the strategy that a predator uses will change as its prey evolves. And vise versa for the prey.