Fleas are so difficult to kill because they have evolved special physical characteristics over thousands of years in order to defend themselves from their host animals, such as dogs and cats.
They have developed a very rigid and tough exoskeleton which makes it nearly impossible to crush or squish a flea. This is an adaptation developed to counteract scratching and biting by the host which attempts to kill fleas in this matter.
Another reason why this so hard to kill fleas is because they reproduce so rapidly. A flea infestation can quickly grow from a single flea to thousands of them in a matter of days, because an adult flea can lay a new batch of eggs daily if it can continually obtain blood from a host animal.
The truth is, with the advent of modern flea control products, killing fleas is easy if you have the right tools at your disposal. Visit my list of suggested products for flea control for more information.