Because they literally copied the D3 code for how that works. In d3 you only interact with other players when you invite them to your group. So, it’s not that big of deal, but in D4, you’re actually always running into other people even if you don’t see them on screen.
It’s a common practice to copy code in programming, but it can lead to be problems if you don’t have people on the team who know or understand where the code came from or why it was designed that way to begin with.
D3 was created well over a decade ago. Most of it’s original coding was done prior to 2012. The original team for that game is no longer around.
The fact is most of D4 was copied pasted together from D3 code in the past 2 years because of developmental delays and mismanagement. The games even use the same engine.
They didn’t put in any effort in creating a new game for D4.