Socrates through Plato, wants to demonstrate how fear can lead to bravery (a distinct dialectical schema of opposites, used by Plato).
That is, fear can lead one to be brave in order to avoid an even greater evil.
However, philosophers, at least according to Plato, have conquered fear through philosophising and accumulating virtue so this schema cannot work for them. They cannot be forced to be brave by fear. If they are brave they are purely by virtue and not out of fear for something else.
Historical note :
Socrates himself, when younger, had taken part in battles and was distinguished for his bravery.
Plato in Phaedo, wants to present Socrates as a hero who even in his last hours is loyal to his philosophy and full of virtue, condemned unjustly by cowards. Plato also wants to show how a true philosopher, like Socrates, approaches death. There is a contradistinction of Socrates, as a true philosopher, and those who condemned him.
Plato distinctly mentions how some friends of Socrates have conspired to help him escape so he can save his life, yet Socrates will instead be faithful to what has stood for in his whole life. A true philosopher.
The central theme of Platonic philosophy and metaphysics, is the eternal realm of Ideas, everything else being in some sense ephemeral and non-real.
Knowing this, the true philosopher, having Socrates as archetype, understands that death is nothing to fear, since is only illusory.
The true philosopher, having conquered the fear of death, has in fact conquered all fears, since the fear of death is the greatest fear.
Thus the true philospher is then brave, not out of fear, but out of virtue gained by true knowledge.