Steam is DRM. Before DRM, this process was solved very easily by copying over files.
What this adds is convenience. Which, I suppose is OK. But, in many ways, "buying games" used to come with some flexibility that we gave up for convenience.
I don't think OP suggest what valve is doing is a bad move. Just that, nostalgically, it used to be quite good. But, nothing stops us from buying games on GoG instead, and just copying over install files the "good old way" either.
This copying concept has actually been possible by hand for some time, whether by using the Backup feature or by way of just copying data from the commons directory of the installation.
In other words, if the install process is necessary, then steam itself acts as a DRM, even for otherwise DRM free games, as you cannot easily copy the install files. Or, at least, not to my knowledge at least.
Before Steam, the game would just yell you to insert the CD after you copied over the files.
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.
This is false. Steam offers OPTIONAL DRM. There are plenty of DRM free games on Steam.