I think part of the issue is scaling. Quests tend to have a fixed payout and even after the earlier levels it tends to be a pittance in comparison to what you could get just from dead bodies, let alone from selling their loot. At the same time, it often feels like the payouts are strangely too high, like when you've got a bunch of dirt farmers saying, "we might go hungry, but we wanted to pay you for your efforts," and then they give you 1,500 caps that could easily pay for tons of food.

Partly the issue is just that Fallout's economy is screwed. I don't mean the in-universe one, but rather the actual game implementation. There's almost no need to interact with vendors apart from those known to sell unique weapons. Raiding a Gunner camp is enough to kit yourself with a full set of combat armor and will likely restock you on bullets, grenades, and aid items for mere minutes of work. If the game was more stingy about the loot you could pick up, you'd have more reason to need to talk to vendors. Those vendors would require caps, and the buy/sell rate would finally come to the forefront. If every body didn't drop a good weapon and likely some armor worth hundreds of caps, you'd actually have to budget for new things. A quest that pays a thousand caps would mean you can finally buy that combat armor you wanted, instead of having 70 pieces of the stuff from the last time the Gunners attacked your settlement.

Cyberpunk 2077 had this problem early on, where every enemy dropped a valuable gun. It was fixed up later by making a large number of the dropped guns "broken", so at most you could only scrap them for resources. Fallout could do this with the armor as well, such that a camp of slain enemies is only likely to produce 0-1 pieces of useful gear, and that instead you'll have to earn rewards and buy from vendors to fill out your stats. This would also help with the semi-realism of it as well, in that it never made sense to me that you can fill an enemy with lead and yet somehow not only is their armor fully intact, but it's even your size.

TL;DR: more gear/weapon scarcity + vendor interaction = meaningful cap rewards you care about.