Why are repetitive lyrics only a problem when specific groups do it?

It's so interesting what kpop fans will come up with to fight about...

If the song is nice I'm here for it.

And I also don't know why everything is a reason to fight in this kpop world ..

Depends on the song.

There may be several reasons for different reactions to repetiveness in songs, off the top of my head:

1. Different expectations. If a group is known for repetitive choruses and their fans enjoy that sound (ex. TripleS are known as the "lalala" girls as most their songs repeat "lalala" as the chorus) then few people will complain if their song is repetitive because it's expected.
2. If the group is currently going through a hate train. For example Illit's reputation is pretty low right now, so with their recent release, people are hyper critical about everything, including the song's repetition.
3. The overall quality of the song. A high quality song can make up for a repetitive chorus (ex. TripleS), but if the song is already bad to begin with, a repetitive chorus sounds even worse and more annoying

RING DINGDONG DING DONG DONG RINGADING DINGADINGDINGDING

Onomatopoeias are worse.

I take a wild guess: this is about a specific group?

LOL, we have an agreement about something finally?

On topic: This is not surprised, Kpop fans are hypocrites, especially the mainstream fans. This is a thing since the beginning. People trashing songs from small or nugu groups, but if their faves release similar Tiktok trash, they say it's SOTY.

This is why the term "fan brain" exist. Most fans can't put aside their fan brain and stop being biased.

Also, the problem isn't really the repetitive lyrics, even if it's repetitive, the composition of the song can make the whole thing much better. Especially if the composer adding some ad-libs or change small things in the Chorus.

A good song is a good song and bad one is bad. If repetition makes the song catchy then they achieved their goals. If they annoy the heck out of you then the repetition was a fail. It's the same for empty chorus or dance breaks. There are good and bad examples of them.