Listen there are many reasons and all of them are low-key bullshit but I wanna give it a go for why I think people act like this
1) Homophobia listen this fandom is old as hell and inspite of a lot of the people in it being teenagers that does not erase history Percico was one of the first really big gay ships and keep in mind PJO came out in 2005 of course some of the hate is just straight up homophobia even if they try and disguise that with some other more acceptable reason
2) self shipping specifically pseudo self shipping that a lot of people do using solangelo and percabeth as vehicles a lot of people really like Nico and really like Percy and the fact of the matter is Annabeth and Will are incredibly easy to project on to so people get to live out their fantasy of being with either boy without it being explicitly self shipping
3) Shipping fandoms have this really awful tendency of when they hardcore ship a character with another they get incredibly hostile when the perceive someone to be encroaching on their ship it's kinda gross but it's just a fact of fandom
4) Some people just don't like either character but love the other and don't wanna ship their fav with a character they hate
5) this is the most often sighted reason, the age gap which I personally find to be the most eye roll worthy because one, Rick's timeline is wack as hell because you either believe Nico was 10 at the start of TTC and would have thus ended HoO being 12 maybe 13 at the most or you believe Percy was wrong in his age assessment and he's 14-15 which is the age Rick tells us he is, in the first case why would he be dating at all and in the second case considering Percy is recently 17 (remember he's still 16 when he gets taken by Hera and the entire thing only last about 10 or so months so he'd still be 16 almost 17 ) in which case the age gap is around 1 or 2 years so hardly worth thinking about but ultimately none of this matters because fandom is by nature transformative meaning the age gap is functionally irrelevant because in fanfic or fan art I can do whatever I want with the ages I can make Percy younger than Nico I could make them the same age I could make them fucking 60 or gods or literally anything the age gap literally doesn't matter so the hang up is bs because the person saying it is either, not that engaged in fandom in the first place, a teenager themselves and thus has doesn't wanna see their favs be older than them, or lacking imagination