XP and loot alone are not enough incentive to spend time and ammo.
The loot in T5 POIs is not much different from T1 POIs. You get a slight loot bonus, but if I compare a T5 POI in the Pine Forest to a T1 POI in the Wasteland, for example, the T1 POI will give me better loot, and in the same time it takes me to clear a T5 POI, I could clear several T1 POIs. And there are more effective ways to get a lot of XP than spending a bunch of ammo killing zombies.
You can also look at it in-character. The quest is a job. Why should I take a job that doesn't pay?
Before there were quests, I hardly ever looted. I find looting very boring. That's why I like quests. I drive or walk to where the trader sends me and do my job. It also allows me to explore the map without driving around for no reason.
That would certainly be an interesting mechanic.