I have 1Gbit at home, but almost never reach those speeds when downloading games. It’s one of those cases where it makes sense (I want to play now!), but I’m under the impression the limit is upstream (at steam most likely), rather than on my connection. (I do get those speeds on speed tests, doesn’t seem to be my setup).
ISPs happily collaborate with and put speed test servers in privileged locations on their network so you will get higher speeds there even if the actual peering to the outside world is much slower.
You might check what region Steam is downloading from (it's in settings -> Download or something similar). If it's selected poorly, you might do better by picking one yourself.
I have 5 gigabit and usually get ~1.2 gbps, sometimes get up to ~2 gbps from Steam.
I get full speed on steam downloads, even set the limit lower so youtube doesn't buffer.
Steam is tricky cause it has multiple potential bottlenecks. The steam cache, internet connection, decompression (i.e. cpu) and storage. Often hard to tell which limit you're hitting