This is surprising to me: I've seen Steam peak around 1500 on my connection. Linux ISOs from cloud mirrors (like DO) can hit closer to 2gbps. Bittorrent downloads of Ubuntu ISOs sometimes hit 2gbps
I think that is more so due to overhead in writing files and most likely also doing some live decompression or some other stuff. After all it does not download single file like iso would be.
It actually isn't all that different. A lot of modern games are basically a smallish exe and a single giant Assets file that's many many GB, and is some sort of encrypyted (or not) filesystem image.