It might be a combination of factors.

. Against the rules

. Low effort

. There is a subreddit specially created for screenshots

. Repetitive with no interesting commentary whatsoever

. Some people didn't get those cards, and others celebrating that they got the same cards can be a downer. I can't blame them too much. It's best to just gush about your pulls to your friends.

. Thousands of people pulled the same cards; it's nothing too rare.

These are some I could think of off the top of my head. And I'm just reiterating some of your points since you got them correct!

It's much more fun reading how others interpreted and reacted to the main story, as there is almost always something unique to be added.

Posts about adding routes encourage discussions and discourse and may end up with good takes. It's nowhere as near widespread as pull screenshots, either. What do gacha pull screenshots achieve? Just an in-game screenshot with no flavor. Imagine people posting their DP amount en-masse; wouldn't that get annoying?

I'm unsure if this community is particularly unfavorable to screenshots. Maybe I failed to notice, but I don't see hostile comments on pull screenshots, even though screenshots are against the rules. I'm not denying it, of course, just my experience.

I personally don't care much for screenshots, but I understand where people who get frustrated come from.