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.
But.....you still can just "copy over files" if you really want to bring over the good old 2002-like flow. Just copy over files from another machine, then start the install process, steam will realize it already has all the files and will finish instantly, done. I've done this many times.
Steam does not enforce digital rights management unless the distributer wants it. Many games that can be executed from their own directories without Steam even active agree with this. Steam is first and foremost a software distribution platform.
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.
The only way steam is not itself a DRM is if copying installed steam files is a facsimile of everything the game needs and expects. I do not think that all "DRM-free" games in steam are excluded from making some kind of necessary system changes as part of its install process. Be that changes to registry, or the likes.
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.
And inevitably CDs would become unreadable or lost and one could no longer play the game. I remember one of my Baldur's Gate 2 CDs having a crack from the centre to the edge after suffering an unfortunate fall.
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 only way this was true was when cracks were a thing. Most games had some sort of copy protection before (and often after) steam. I couldn't imagine a community of idiots downloading untrusted exes these days...
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.