Because D3 is 100% instanced. You only see others in a party.
D4 is more open-world. You see others regardless if you’re in a party or not - except in dungeons, which still use the instanced design. The main issue with D4 going full open-world is the lack of infrastructure to support more than 12 players in a given area.
I’d bet that D5, when it arrives goes full-on open world design, save for party only endeavors similar to Citadel (or raids if you wish to label them as such).
The days of single-player Diablo are over - even if D4 players prefer to play solo in the quasi open-world. You can see it clearly going from D1 → D2 → D3 → D4. D5 will lose some players doing so, but the truth is D5 won’t be made for you.
Just sad, hard facts.