Because most of the software people run is compiled for x86-64. I don't know whether the thing about the App Store is true, but even if it is, the App Store is

Because most of the software people run is compiled for x86-64. I don't know whether the thing about the App Store is true, but even if it is, the App Store is not the only way to download software on macOS, and in fact I suspect only a small minority of people primarily get macOS software there.