I want to understand why people ship these two, so I've thought about it for a while. I've seen people ship KuroKura for a few different reasons. They ship them because of the "Enemies to Lovers Trope", age gap relationship, for the angst, because they're two conventionally attractive men, and/or because they "ship" them as platonic rivals/mortal enemies. While not all ships are romantic, shipping culture puts an emphasis on romantic relationships. Some simply enjoy KuroKura as enemies, which is why they're often paired together.
Remember, shipping means to pair two characters together, not always in a romantic context, but it usually is or has romantic undertones. I find KuroKura to be disturbing, but I know many enjoy more toxic dynamics or like the idea of seeing how enemies navigate a romantic relationship with one another.
As for the age gap, people assume Kurpikra is 17, refusing to acknowledge how characters age in the manga, as it's still ongoing. Kurpika's birthday is April 4th, 1982. He was 17 during the hunter exams, but two-ish years have passed since then, making him 19 in the manga. Shipping Kurpika when he was 17 with Chrollo is a pedophilic ship, making it a pro-ship. If you're shipping him with Chrollo when he is an adult, then it isn't pedophilic.
I do find their large age gap to be weird, but I don't think age gaps between two adults should be seen as a big problem in fanfiction. Most people that I've interacted with or have seen ship KuroKura do not want them to ever be canon, but do enjoy the hypotheticals of seeing how the two would work together. While I don't ship them, I don't mind if others do as long as they aren't being weird about it.
I do want to talk about my problems with KuroKura fandom and why I don't like them in future posts. As of right now, I won't dive into it much, but I will talk about Chrollo having killed Kurapika's clan. This is the main reason I don't ship them, and something I see people complain about when it comes to the ship. While I understand people enjoy problematic and complex dynamics, this one isn't one I can ever see happening in canon, and most Kurokura shippers agree on this.
While Chrollo, I don't think, is shown to hold any animosity towards Kurpika, seeing him as a threat to the troupe at most, Kurpika, on the other hand, holds a deep-rooted hatred towards the Phantom troupe, aka Chrollo. This has great potential for an angsty fanfic and depiction of a complex ship dynamic. However, this should never be something that is generally romanticized. Which is why I understand the hate for the ship, and why I don't like it. However, for me personally, this isn't enough for me to really want to see the shippers be persecuted for those who like it. At most, I just block them and move on if they're causing problems.
With this being said, I would like to see more Kurokura ships have them work through their hatred and in the end go their separate ways. Allowing Kurpika to truly feel the grief of his clan and Chrollo to gain a sense of humanity. This is unrealistic for their canon character, but for a fanfic, I would adore this. This is the only time I'll ever be caught enjoying the ship, as I love "doomed relationships" that end with the two not being together.