It's kind of a symbiotic relationship if you think about it, similar to the situation in most of the "Tom and Jerry" cartoons. The reason Tom doesn't ever really TRY to eat Jerry is because Tom knows that without Jerry, he'd be out of a house and job. And the reason Jerry doesn't ever really TRY to get Tom killed is because Jerry knows that, A. Life would be too boring without him and, B. Without Tom, whoever is/was Tom's owner might actually get a pet that might HAVE no qualms about eating Jerry. It's the same reason why Mr. Krabs has never thought of to simply crush Plankton with his feet, Life would be too boring without Plankton and without Plankton, there might come along someone who's even BETTER at trying to steal the Krabby Patty formula than Plankton was. I just kind of wanted to make this analogy in order to get people to see just how this kind of relationship works in a situation similar to the one that is seen in most of the "Plankton and Mr. Krabs" episodes of "Spongebob Squarepants." Enough said!