Why is upload more expensive than download?

I don't get it. Network transit and cloud providers generally charge (a lot) more for upload vs. download traffic.

Why is this? Making a couple of assumptions, but surely at the network device level, the chips that send/receive traffic are almost identical. They are doing the same thing just in different directions, so why would one be more expensive than the other?

At the media level, I'd expect it to be the same. A piece of cable or fibre can send data either way and doesn't become more expensive to use in a certain direction.

I can't think of any extra costs that need to be passed on from handling upload traffic, other than you are then accepting more data into your core network and so need a better (more expensive) core, but then the same could be said for downloads?