I suggest looking at the gamedevs and publishers to get a rough idea of what's happening behind the scenes because you unlikely to get an official response from any of these companies.

The newer games from 2010-current published by SE are the Deus Ex series, Life is Strange, Goetia, and Fear Effect: Sedna.

Deus Ex and Life is Strange are managed by SE Europe. Gamedevs are Eidos Montreal (SE Europe) and DONTNOD respectively. Notably, a lot of their older titles are on here like the old Tomb Raider and Thief series. Goetia and Fear Effect are managed by SE Collective. Gamedev is Sushee for both. The inactivity of their website and social media could suggest they're defunct now.

[Speculation] This suggests SE Europe has a say in getting their games published here. And that the original gamedev has a say in getting their games published on platforms regardless of belonging to SE Europe or Collective. Unlikely to see SE Japan's games like FF here because it's tough to convince their business to drop DRM on their games. Their businesses are still very traditional and are very protective of their IP. Some exceptions do exist, but they're in the minority like the ones published by Ghostlight, XSEED, NIS America, Retroism, DANGEN, Playism, Aksys, SNK, etc.

Reading through these academic papers from the following, it appears:
- https://www.sciencedirect.com/science/article/abs/pii/S0167923609001341
- https://www.sciencedirect.com/science/article/abs/pii/S0167624510000491

1. If DRM has been cracked and/or there is strong copyright enforcement, DRM-free is more profitable for the gamedev / publisher

2. Implementing DRM when DRM is effective or when copyright enforcement is weak (usually always online)

3. If the gamedev is the publisher, the'yre more likely to release the game as DRM-free

4. Premium quality and newer games have more DRM

In general, if companies like SE or Bethesda perceive their in-house games as having weak DRM, that countries of their major demographics are actively pursuing pirates, and their games are very premium and worth their asking cost, it's more profitable or beneficial to them to always have DRM. For the DRM side, I speculate companies like SE and Bethesda perceive #2 and #4 as huge barriers to overcome before considering to publish their newer games on GOG. For older games, then #2 still remains the larger obstacle to overcome.

The other main reason is the GOG market isn't big enough to be worth it. GOG comprises of 1.4% of the PC digital distribution market. So there aren't a lot of economic incentives for companies to publish here. And this is besides the aforementioned required work to maintain the offline installers.