A lot of it is probably from salty fanboys from the 'other team'. A saw a bunch of them shit-talking about Horizon yesterday on Twitter (focusing on the US Gamer 2.5/5 review, of course) and I tried to engage with them and ask why they hated a game they hadn't even played. Of course I didn't get a clear answer aside from Horizon just being a copy of The Witcher 3 + Enslaved, but we did also get to a bunch of other PS4 exclusives being crap, like Ratchet just being a remake, Until Dawn having a low framerate and just being a movie, Bloodborne being just a Dark Souls copy which is shit compared to Ninja Gaiden (?) and my favourite of all, he didn't like The Last of Us because it promotes homosexuality!
I realise I shouldn't have bothered and I know engaging with devout fanatics is pointless, but I was trying to engage and see why they felt so threatened by this game. All it made me realise is that they are completely incapable of any rational thought, and more specifically that Horizon has rattled an awful lot of cages.