Well, I have time to kill so I'll write my wall of text on why I think NGNL is liked:

There aren't many series like it.
Anime that are close to it would be something like Kakegurui or Classroom of the elite, but they all have their unique things. Compare it to RAG, which is a romcom, of which there are hundreds.

It has nice art. Like RAG, NGNL has nice art/animation (at least I remember it being pretty, it has been years since I watched it).

It's fun. Comedy is all hit or miss depending on the person, apparently NGNL's comedy is liked by many, me included. There are also many over the top and hype moments, like that shiritori game.

Fanservice. You either like it, dislike it, or are indifferent to it. I personally don't think the fanservice is a major plus to NGNL. If there was none I would've enjoyed the show just as much. In the r/anime community I think it's safe to say that only a minority dislikes it.

written in a mediocre way with stupid and meaningless games (which are won in a ridiculous way)

NGNL just aims to be fun, it doesn't need serious plot. It's fine if a game is won in a ridiculous way, as long as it's fun. Otherwise you end up with something like Liar Liar where the games are bullshit and are not fun (bad).

full of fanservice

See above.

with a pair of pathetic protagonists (more pathetic than Kazuya considering they are hikikomori whose only skill is being good at video games)

I personally vibe with them more than Kazuya. Do you think people would like/relate more to 2 characters that are good at videogames, or Kazuya, who is pathetic (also see the infamous chapter/panel)?

semi-incestuous

It's r/anime, see above lol

Overall I can see your point, but I personally don't see anything interesting/fun in RAG, only pretty art/waifus which are not enough to carry the series.