The problem is that with Steam you only know if that works after you bought the game and only know if that works across machines if you upfront have two machines to test it in.
I mean, if you know upfront that it matters to you (which you might not until, say, your machine breaks and you happen to have no access to the Internet or Steam in your new machine yet, at with point you'll be thinking "I wish I checked") you can go through all the hassle of always thoroughly testing it within the refund period of that game, but at that point piracy is less of a hassle.
Meanwhile some of my GOG offline installers are so old that they have been used on 3 different machines (well, one was the same machine under Windows and under Linux) already.
Don't get me wrong - I use both Steam and GOG, my point is that saying that "Steam has DRM free games" is even worse than a half-truth and about as bollocks as saying that a shop selling TVs is selling "Quake game machines" - sure, people with the right skills can get Quake to run in some Smart TVs, but that's not how the store is selling them as, that's definitelly not supported by them and they won't refund you a Smart TV purchase as "not suitable for purpose" if that device fails to runs Quake.