Because it's not the immediate root issue. The problem isn't with the current size of the population. It's resource distribution (rich versus poor, developed vs developing, etc.). Someone somewhere calculated that the planet could theoretically support 9-11 billion people, if everything were equitably distributed.

And by the time we hit 10 billion around 2050 ... well, let's hope Elon Musk has taken us off-planet by then and into the solar system.