The reason is pretty obvious. By "exposing" yourself elswhere you have guards, mobs, and an actual reward for achieving your given quest. Reward in tokens that will eventually lead you to buy stuff that will also be usefull in PvP. Not the same context.
The problem isn't digging. It's sustaining a fight. Seems that AoC players aren't able to do anything that doesn't XP anymore so that aspect rules... The digging is the trigger. Bori is suppose to be open-world, spontaneous, groupe based PvP. You need that trigger to have some of that. You need players to be somewhere doing something to trigger that.
Now the concepts that failed. Altars were suposed to compete with each others. Meaning instead of just having one active altar at a time you should have had up to four. Meaning you needed a portion of your groupe to defend your altar, an other to dig/loot from ennemie groupes, and the last slowing down, trying to cap ennemie altars. A paradise for hunters and coordination freaks.
That's how you were suppose to gain XP while fighting. But instead players destroyed the looted mats and never had altars compete. Because that's what's bloody intresting to achieve. Actually managing to dig while harassed by other players. And if this was actually what players did it would help sustain fights because even if you lose the upper hand you can still recycle yourself in hunting.
The mecanisme Funcom should have deeply looked into is how to make the fights there last and still be guided by the mechanics of the zone. To keep the fight's intresting and as far from rezpad as possible. Digging isn't the issue not to mention viable without being necisseraly bipolar