They would be terribly inconvenient though: they don't stack easily!
Imagine carrying a couple of thousand bottle caps around
Yeah, one bottle cap doesn't weigh very much, but a couple thousand seems like it have to weigh you down quite a bit. (Though, you know, you can also somehow manage to fit an unlimited amount of ammo and a fatman in your
pockets so.... (The Vault Suit is a revolutionary design, incorporating a magic bag of holding, don't you know.
)
And then imagine that you want to buy something that costs... say 874 caps... that you'd have to COUNT! And they don't stack, so you might have to count them one by one!
That at least is probably not so hard. I don't think it'd be too much of a stretch to figure that the merchants you're dealing with use a scale to count all your caps. As far as stacking, one possible rationalizing is that instead of just having a huge bag of caps jingling around - you could probably easily string them together with some thread - maybe making a knot every 50 or 100 caps or so; so you could keep track of how much you have. That's what I'd do, at least, if I wanted to keep track of how much money I had. (I could be wrong, but wasn't that kind of how they did it in Feudal Japan, with their Yen? They had that square hole through the middle...)