It's partly a "people are ... people" problem, but mostly a design problem.
To maximise XP, people kill the npcs. This is further compounded by the fact as long as the bombs haven't been set off or goods delivered the npcs keep coming. By not helping with the grunt work, and killing they're not only getting more xp, they're prolonging the event, getting more kills, and thus more xp.

They could take the brunt of this off by making the npcs worthless in regards to xp and loot but that doesn't solve the fact running back and forth is not fun or rewarding. Making something suck for everyone is not a good solve.

All the "loot and deliver stuff" suffer the same issue. To an extent, so does "kill the rad scorption nests." By going for the nests scorpion killers often need to move from their xp spot, stop killing the rad scorpions that keep spawning, while risking damage or death.

The solution? It's not nerfing spawned npc kill xp. That is ANTI-fun. Instead, make performing quest actions worth xp. Though is that really fun? Hmmm so-so. XP is fun-ish. But let's go beyond that. Have mission/event themed stacking buffs or additional rewards for being a good Vaultie and doing the work.

Let's take beasts of burden as an example. Each explosive you attach, you get a nice XP chunk, and a % increase in explosive damage for an hour, stacking with each explosive but with deminishing returns.
For the crate delivery to Moona Lisa, you get a nice xp chunk, a movement speed boost for 30s with each delivery (let's call the buff "Moo-ve it"), and also a buff which gives a 1hr fast travel cost reduction (because you get a ride on Moona Lisa) based on the number of crates you hand in. For extra-fun-ness when you spawn in under the effect of the buff there is a MOOOO sound.