You can manually copy over the game files even with Steam. That has always worked. Steam even offers an export option in case you don't know how to find the directory yourself.
The problem isn't the copying part itself, it's getting access to the files. Windows network sharing isn't enabled by default, modern Windows even disables network discovery by default. Even when you enable it, you might not be able to access another Windows computer if it's running a passwordless account.
What Steam is doing is automating the access control. No longer do you need to start configuring your OS accounts and setting up network shares. Now it just works.
The problem isn't the copying part itself, it's getting access to the files. Windows network sharing isn't enabled by default, modern Windows even disables network discovery by default. Even when you enable it, you might not be able to access another Windows computer if it's running a passwordless account.
What Steam is doing is automating the access control. No longer do you need to start configuring your OS accounts and setting up network shares. Now it just works.