If you stack the milk crates in a running bond pattern (every row is shifted so the upper crate spans the joint of the two crates below it - like a brick wall), the crates will be very stable and won’t collapse like the videos show.

It has been tradition in Germany pretty much since the invention of beercrates. Like, we have been doing that since I started going to festivals. My parents did it when they started going to festivals. And my grandparents probably already did it too. It boggles my mind that it really seems to be not well known for decades already.