tfishell: We get plenty of modern games. What you're talking about are AAA/big-budget/"premium" games and publishers almost never want to give us those because they don't want them DRM-free and GOG is very small so the revenue is not worthwhile for them.
The gaming-community, such as everything else, is driven by "trust" of the community. Steam was in many terms ahead of GoG and so they had the biggest amount of "trust earned" from the community and especially from devs who love DRM as Steam is a DRM-model, with one or even several DRMs (third party). So, there is 2 main supporter at the 2 end playing a huge role here: Customers and Devs... both are enjoying it in many terms.
However, GoG was the first with a new way of marketing, getting ride of unwished DRM methods, and the love for the "Good old game", so there is a high focus on something kinda lacking on Steam. Aswell a "free to use" launcher (some goodies involved yes, but it is free to use). In the beginning this new "marketing" was not very well supported by either the community nor devs, because gamers was satisfied with one single way and one single platform. However, over time GoG was able to earn a high respect, not only gamers... industry aswell. Nowadays GoG is the second largest marketplace for PC games, this is simply a fact. To GoG it is not very rewarding because they are demanding for a lot using a DRM free way. Especially big studios could "care less" because their image is allowing them to get "ride" of their game at almost any spot and always for special conditions. More might simply means more choice and better offers to them, so they can be picky.
However... the DRM itself is a big issue but not the biggest... maybe one of 2 big issues, simply not the only big issue. Another big issue is "there is simply not enough revenue" for supporting a rather difficult platform demanding for a lot of custom approach (because of no DRM of course). So they may think "OK, we may consider GoG because of customer demand, but it will not be our premium... just an option and to make new customers happy".
Finally i think, GoG, as "the second force" and with a unique valuable approach giving some new legitimations and valuable competition, was never that strong such as of today; so the future actually looks bright. Remember, it all got to do with trust and acceptance, from gamers and industry aswell... and nowadays GoG is at a spot able to seriously take part for some serious matters, even for "premium-developers".
Epic Game Store on the other hand is a third option no one truly needs because apart from "many free games" there is not really any additional value to the customers... to be honest. It is a third force simply unable to provide a special value, except the cheap way of "here you have another free game" which is somewhat just stupid... sorry. However, i do respect Epic Games for their surely awesome game-engine (UE5) and nowadays even GoG is now closely connected to Epic Games to some extend. However, i still hope GoG will stay unique and largely independent from any other "force" in this industry, so they can stay the way they are and how the community want it to be: Authenticity and trust is critical.
However, there is no clear evidence what actually counts or matters as "premium game"; it can be games made with any size and any budget able to become "premium" and some very big budget games was actually pretty bad games to a huge amount of customers... so big budget doesnt automatically mean "premium".
AAA on the other hand is just a fairy-tale, trying to provide a certain number of a budget, although in many terms we do not even know the exact budget of a game, so there is a lot of guessing here. It should not be able to "rate a game" and mainly become a matter of "investors" who are seeking big numbers. To the customers, the meaning should be low or not truly relevant at all.
Still, fact is, many "big budget games" are usually either not released on GoG or released very late on GoG. Because those publisher either got in mind "not truly worth it dealing with, we get enough of revenue with the exclusive Steam offer" or maybe even Epic Games providing a almost free way selling their games. So they may feel not much use dealing with custom builds without DRM. Time may slowly change i assume, it all depends on the customers not blindly accepting any "bad conditions" on Steam, for example Denuvo-DRM (unfortunately still with high acceptance) or other limitations and hassles. If they do... because of the "new hype"; DRM is playing a big role at the launch-sales aswell... because it will generate a huge revenue to them; all what counts.
At least in the past... i got the feeling, nowadays the gamers are acting more picky when it comes to "bad conditions" because nowadays they know "there is a second force possibly able to provide a even better version", so the "big companys" may consider a release more than ever before.
Stil, there is now many "big budget" games on GoG, at the current time, and it will surely increase even more. I do not think the situation is bad but there is always room for improvement, this is certain.
Fact is we got 3 true PC marketplaces and 3 true console-marketplaces... and GoG is nowadays at the second spot (in its category) thanks to the customers and increased trust from the industry aswell; there is much of room for new hope and new improvements. Actually, at the time GoG was starting, i never would have guessed they are able to grow that strong by selling "Good old games" and DRM-free. The idea was somehow "out of this world", simply almost unable to become realistic.... but they truly succeeded doing so; this is something always to be considered: It is a difficult way but a own and unique way... and of course in need of steady support from everyone else.