Why does it take so many people to make a video game?

AAA publishers often complain that it costs too much to make a video game. Take Diablo as an example: Diablo 4 had 8,500 people working on it, while Diablo 2 was made by only 70 people. Yet, despite the huge difference in team size, the gameplay evolution isn't that significant. While Diablo 4 has better graphics and more voiced content, the core gameplay hasn't changed much. Technology doesn't seem to be the issue either, as smaller teams like Supergiant Games can create successful games like Hades with only around 20 people. So, why employ over 100 times as many people if the end result isn't significantly better? Is it all about better graphics?