MysterD: These two (EA and Ubi) both have monthly sub-services on their respective services. They keep stuff on their backlog there to give it more value.
SargonAelther: Sure they probably do think that way, but this mindset fails to account for people who:
1) Only buy DRM-Free.
2) Have already bought these games on UC / Steam / Epic, but would GLADLY re-buy them on GOG, because they value DRM-Free games more.
Therefore publishing more old games on GOG will not lose Ubisoft potential money. It will earn them more actual money.
Also many Ubisoft games sell well on GOG, so much so that Ubisoft sometimes even gets their own section on the homepage. Ubisoft games often sit in the top 10 recent bestseller list and HOMM3 is the #1 bestseller on GOG of all time. I think if Ubisoft published more of their old games, they'd sell like hotcakes too. Ubisoft needs to realise that just because UC and Steam sales may have dried up for old titles, that does not mean that the same will apply to GOG, because GOG offers something that "nobody" else does.
This is the thing: they only care about their mind-set. That's my point.
They also probably want to cut-out any of the likely standard 30% fee or whatever GOG takes, when they can instead take 100% from UbiSoft Connect via game-purchases (and I use that term loosely for game with DRM/client-app requirements) and/or UPlay+ Subs (or whatever they might be calling their subs now).
I doubt Ubi cares much for the small percentage here on GOG here, since it likely ain't doing Steam-like percentages of sales here in the PC gaming digital download market.
EDIT - Ubi is say not Sony. Sony wants to hit every avenue they can they ain't Microsoft-owned walls b/c they like to sell their games wherever now - i.e. PS4, PS5, PC (except Microsoft Store on PC and Microsoft Xbox console). Even Sony is bringing their stuff to GOG, of all places too - which is great and I love to see this! They seem to care more about actual game-sales than say game-library monthly subs and/or yearly subs, unlike say Microsoft and Ubi.
Sony didn't buy Nixxes (expert PC-porting company) for nothing.
EDIT 2 - About HOMM3, there's nothing for Ubi to rework here to get it onto GOG. There's no Achievements, no client-app requirements, or any of that junk from the old-era. They can basically more or less straight port that to GOG, Steam, anywhere on PC without nobody really caring about any of that modern-stuff. That would be the case for any Ubi games since AC2 that required Uplay/UbiSoft Connect, as anything since then (except Prince of Persia: Cel-Shaded on PC) was laced w/ that junk.
And PoP: Cel-Shaded Reboot (2008) went to retail without DRM and didn't sell well. Ubi said they'd try it (DRM-FREE) b/c gamers big-time asked for a Ubi game without DRM at retail - well, this sadly proved Ubi's point, so they went to releasing junk with Uplay. Nobody really bought POP 2008 at retail. Probably the only reason that's one of the few PC versions of somewhat modern Ubi games around AC2 era on GOG b/c it was painless and easy to port b/c it was DRM-FREE and bare-bones. Nothing to re-work client-app wise (it wasn't there), no need to implement Achievements (that stuff wasn't super-common on PC everywhere yet), and/or anything of that modern sort.