They're both to blame. The only reason you can't mix & match base games vs DLC same way you could buy Oblivion & Shivering Isles discs separately from different stores and have them 'just work' is tying them to clients then have the client "discover" DLC instead of the game. Steam version will refuse to accept DLC that doesn't have the correct Steam AppID "authorized" the client, so they're obviously to blame for their own DRM, but GOG just copied them in wanting Galaxy to activate even DRM-Free DLC versions via its own special Discovery API method. Delete the goggame*.info file from some such games who use it and the DLC will "unactivate" as if it were never even installed (despite the fact the DLC files are still present, it's just Galaxy metadata you've changed).

So the honest answer is - Valve created the original mess in wanting 1001 features that should be kept in-game unnecessarily "managed" by the client (which just like store-specific achievements 'accidentally on purpose' multiplies the developer's workload for every additional store they want to sell on and have to rewrite code for), and GOG, Epic Game Services, etc, mostly just passively follow the lead. Prior to this, games would detect their own DLC in-game just by looking for the files locally or have the DLC replace / patch the "base" game's .exe with a DLC-aware version, and in fact many pre-Galaxy games like that on GOG do still work that way. Eg, I can take ARMA : Cold War Assault (GOG version), add the missing Red Hammer DLC from Operation Flashpoint : Cold War Crisis CD-ROM GOTY version, and as if by magic it works (precisely because it isn't actively managed by an over-bearing client)...

Indeed they both are, and also the publishers. The reason why, though, is not clients - but maxmising reveue streams. There is no other consipracy here than normal human greed

Ok, someone tell me please why I helped this guy with Linux related advice in other threads?

He clearly did not deserve my efforts to input 1s and 0s through my keyboard.

He can't be serious.

That's right and pretty normal, as it is looking for the right game, and nothing to do with DRM.
Though you could say, the Steam version is the problem, as it probably has DRM.

A GOG version of a game is not the same as a Steam version of the same game, just as any DLCs made for the GOG version would not be the same as DLCs made for the Steam version.

While there are some exceptions, DRM-Free versions don't tend to work with DRM versions.

All pretty logical really.

Since you can't purchase any DLCs here on GOG, without also owning the base game here on GOG, I'm gonna assume, that you own "AoW 4" on both platforms: Steam and GOG.

And you (wrongly!) assumed, you could buy the DLCs just once (DRM-free here on GOG) and then use them on both platforms.

That's of course NOT how this works, because every store wants you to spend your money with them - not with the competition.

That you can't use the GOG DLCs with the Steam base game, doesn't mean the DLCs are not DRM-free.

It simply means, that the DRM-free GOG version (of the base game AND its DLCs) is a version of its own.

Same with the DRM-ed Steam version, which also is a version of its own.

Or the EPIC version.
Or the PS 5 version.
Or the ...you catch my drift.

Except that OP is just low effort trolling. I don't think it is fair to compare our high effort trolls to him.