Breja: Once Galaxy becomes mandatory, this thread will be the best answer why. With how many people here use all those clients, there's really little to no reason for GOG not to make Galaxy mandatory.
Desmight: Most people don't care about ownership, preservation and privacy; as long as they can play the latest game they saw on Twitch, they're happy to make 10 accounts and install 15 clients. It's sad, but that's the truth.
Also, a lot of those clients/accounts have been used to redeem free games (Origin/Uplay/Twitch mostly), it's not like people actively use them.
I very rarely watch anything on Twitch. And when I have it's usually just been for friends.
So I can pretty much guarantee that my gaming has not been influenced by twitch.
As for privacy, I could give a rat's ass who knows which games I own. I mean unless you are some recluse, your life is not private to the public. People see what clothes you wear. They see what vehicle you drive. They see what you buy at the store every time you shop. They see how you work out every time you go to the gym. They see you. Your face. Your attitude. Now again, you could be a recluse and they see none of this.
Now privacy for me and most people is an issue when it starts to involve things that actually do matter. Like your full name, social security number, credit card information, home privacy, and other things being invaded or leaked. None of those clients have ever affected those things for me. And any client that has ever had the option to disclose your full name to the public has always had the option to hide that information.
And as for a lot of accounts being set up on other clients, I can agree some are used for free games, but most are set up for games that are extremely popular (not because of twitch) such as on uPlay having Far Cry and Assassin's Creed, and Origin having games like The Sims, Sim City, Battlefield, Titanfall, etc... You may not like those games, but quite a lot of people do. Hell Titanfall 2 is one of the best FPS games I've recently played. And you need Origin to play that. You'll never see it on GOG.
So why would I use GOG if I use all of these other clients? Well GOG has a ton of classic games I will probably never see on any of the other clients. Games I enjoy.
And you see there's the thing. If you have people who actually like games, and not brand loyalty (fanboys), you're bound to have people with multiple clients. Multiple platforms. Multiple consoles. You know, not just do the fanboy thing.
edit: And before anyone says "But the scammers!" I have had my game library on GOG public since it became an option, and long before that I have had a GOG wiki which I constantly update with my game library, also public, which actually links to some of my other accounts (e.g. Steam), and have never had anyone attempt to contact me. Now on Steam they have had and will always have issues with scammers (same as anywhere else, including GOG I'm sure). I have actually been victim to a few scamming attempts. Keyword: attempts. And the last one was probably 6 or 7 years ago. Steam has made a lot of improvements in fixing this with a ton of safeguards. And I have not had anyone contact me in many years to attempt anything. Despite having over 6,000 games (which can be viewed), and a few hundred games in my tradeable inventory, including quite a few removed titles. Most of the people still getting scammed are actively putting their info out on trading sites and groups. Just having your game library public is not a guarantee to get scammed.