People harp on about Kairi because of the trio formula that Kingdom Hearts and other games/series in the same genre has stuck to for so long.
You got Sora, Donald and Goofy. Mickey, Donald, and Goofy. We got the Sea Salt trio (like em or hate it). You got the Wayfinder trio (like em or hate em). In Union/Cross/Chi you've still sorta got a trio formula going on. They keep introducing NEW trios. Axel, Saix, and probably Skuld or whoever Subject X is. Marluxia, Larxene, and Streltzia potentially. Yozora and his two bros trying to save "Stella". Of course, not every group follows that formula (namely Riku and Mickey) but it's almost a staple at some point. People try so hard to fit characters like Namine, Vanitas, and Repliku into this formula too (doesn't always work out but I appreciate ALL trios, even fan ones)
After all the focus on other trios for at least two games, it's understandable why people developed this "Alrighty, now it's Kairi's turn to step up" mentality.
I love a good trio and all the various dynamics that can create. The story really does feel like it goes out of its way to make sure the other two OC trios (sea salt/wayfinder) are.... "together" even in spirit. They're a unit. They fight together. They have each other's back. They have unique relationships with each other but they're all friends. They are legitimately triangles 📐
But the Destiny trio? .... It's completely collapsed and fallen apart. And hey, that's all well and fine. We all love a little broken friendships that can maybe be rekindled one day. But... this isn't that. This is "Oops, we forgot about Kairi again. Hm, I guess we can insert her here for 5 seconds and then find a BS excuse to take her back out again. There we go."
They don't go out of their way to connect Riku and Kairi like they would for the other trios (thinking of the Axel and Xion thing, KH3 did a lot to show he did care about her a lot). Aqua says Terra and Ven's name every 5 seconds like it's a mantra, neither is ever forgotten.
It's so easy to forget that Riku and Kairi were childhood friends, supposedly pretty close, one of the reasons for Riku wanting to see other worlds, and also fuel for a dumb fued between Sora and Riku once.
People have built expectations for the Destiny Trio that the writers... do NOT seem to understand or care about. At this point, I honestly believe they don't see any value in her as a character beyond being a damsel. Although Namine lacks screentime, she still gets so many important things to do behind the camera. Her "Thank you" waiting list is about 5 miles long.