Ancient-Red-Dragon: The reason given on the OP as for why he bought the HZD on Steam is one of the main reasons for why big publishers almost never release their games on GOG, and when the few times when they do, it's only years later,
after those games have become old & obsolete, and their sales have dried up on Steam.
Because the big publishers know that most GOG customers are
not "No GOG, no buy!" types; but rather, they will happily, or at least reluctantly, buy the game they want on Steam if no GOG release seems to be coming.
Therefore, the publisher collects the same amount of money by not releasing their games on GOG as they would have if they did release their games on GOG.
Furthermore, the publisher also eventually gets double-dipper revenues from a lot of GOG customers who will buy the game twice (once on Steam or EGS, and once again on GOG) if it is released on GOG only after it's become old & obsolete.
For those reasons, publishers have zero incentive to release their good-selling big games on GOG before they've become ancient, and oftentimes, even then, they still don't.
I like to use data from the Stealth release of Not Tonight
This game got huge hype, and all indicators was it was going to be Steam only. Dev were very non committal about a GoG release.
When it release it got 1.5-2% sales via GoG
Compare with Divinity Sin 2, Straight from first press release "YES THIS IS COMING TO GOG", GoG was always shown and mentioned on trailers. Everyone knew upfront this was coming to GoG. As a result 8% of sales were via GoG.
So while a day 1 GoG release will make 8% of your customers happier at best its giving you 2% extra income.
For some companies that first fact is more important than the second. For others, that second figure is all that matters.