Because the shape of the population pyramid is changing. From a wider base when the fertility used to be high to about everyone surviving to their 80s. It's this momentum that will keep the total population increasing a couple of decades.
Read Factfulness by Hans Roslin if you want to learn more about this phenomenon and similar questions about poverty in the world.
Read Factfulness by Hans Roslin if you want to learn more about this phenomenon and similar questions about poverty in the world.