Andre Drummond is currently averaging the most rebounds per game since 1970-71. But he could be better.
For most aspects of life, it is important to continually work at getting better at something. Whether it's cooking, building or drawing, it's not going to be perfect right away. Your first steak might be good, but it won't be great. Eventually, you get better and learn how to cook the steak you want.
Andre Drummond is a good rebounder. But he's still a sous chef, not yet at the executive ranks. And the fact that the league's best rebounder could get better should terrify everyone in the NBA.