Why are you not a nihilist?

For the same reason why I don't just lie down and stop doing anything at all. Knowledge of the fact that there isn't any ultimate meaning doesn't change the fact that there exist things which I find enjoyable and valuable. The part of my brain that primarily finds things interesting and valuable isn't wired to make its decisions based on that kind of abstract knowledge.

Why are you even reading this comment? :-)

What is the refutation of nihilism, in a universe made of atoms and the void?

"Sure, there is no ultimate purpose, but so what? I don't need an ultimate purpose to find things enjoyable."

why have the philosophers not all been fired and philosophy abolished?

Philosophy is the study of interesting questions, and nihilism hasn't succeeded in making things uninteresting.