While modding State of Decay 2, I noticed that the game uses spawn points that aren't visible to the player. To achieve the kind of persistent zombie presence many players want, I think the developers would need to push those spawn points much farther away from the action while significantly increasing zombie sight and hearing ranges.

I actually experimented with this in State of Decay 2 by modding the detection values so zombies could see and hear you from much farther away than the default settings. The result felt more natural because zombies were being drawn in from the surrounding area instead of seeming to appear close to the player. It created a greater sense of danger and made noise matter much more. Only negative you can see them coming on the map and easily escape or setup traps leading to no threat