Take in the first two.
1) Most people want a feeling of progress in a game, that the things they do, lead to accomplishment.
2) Because of the above, they will endure a surprising amount of hardship and repetitious activities.
Most people will then see why people do and don't do certain things in 7D2D:
A) Play below ground and build huge underground complexes and long subway tunnels.
-or-
B) Play above ground and gimmick an above ground fort to take as little damage as possible.
-or-
C) Cheat up a bunch of ammo and several "playdough" durability weapons and go crazy.
B is viable if killing zombies has a point. Ether a chance of good loot, experience or clues. As the game provides no real benefit to killing zombies, the intelligent, non-cheating person makes their base below ground.
Same thing with exploration, or as I call them: "Brass expeditions" as that ends up being the ONLY point.
So with a game that completely stops creating fun by mid game and requires the player to make up some fun, or stop playing. You would think that a robust adventure system is at the top of TFP list.
Is it?
Completely disagree that this is the root of the problem. As I said above, I strongly think that a survival game should focus around survival, lest it stops being about survival and turns into a mere mmo-style shooter. One thing is for sure, it cannot possibly be everything at once.
Every action needs a motive - I completely support that. But there are many different kinds of motives with different levels of motivation power that make the player act differently. I play with 6-7 rl friends of mine every time the game releases an update. Observing them, their playstyle never changed significantly since A1, until A15 (if im not mistaken) with increased zombie xp was released, at which point they clearly viewed enemies as grinding material. As enemies are abundant and instantly accessible, they felt compelled to "power level", ignoring their characters' lives/well being or other activities, for the sake of these xp rewards. Completely natural, since the game encouraged them to.
Zombies awarding experience will not make "underground players" live above ground and in the case they do, it will be for the wrong reasons. Players living underground are not a problem in the first place - the underground itself is, according to the opinion of those of us who want underground threats (for our sakes ofc).
As for the brass expeditions - they will just turn into xp grind expeditions. They are brass expeditions because survival mechanics are flawed. Maintenance and upkeep costs need adjustments, spoilage and machine failures need to be introduced, loot needs to be adjusted, events and other more compelling and less bland reasons than xp grind.
But I bet an arm and a leg, that many of these problems come from the plenty of options we have in the first place, that are supposedly always a good thing. Hopefully someone understands why I am saying that but won't get into it further at this point.