That's just because right now used coffee is trash and sand isn't. If you remember back when bio diesel started getting popular, all of a sudden people were stealing fryer oil from restaurants. If you see a smelly looking black dumpster behind a restaurant, that is the used fry oil.

But still, it’s strange to think that this exotic bean which will only grow in certain climates can actually be easier to get than fucking sand.

There's different kinds of sand and concrete needs specific types to work well.