Because people like Markiplier and Jack atleast sometimes reign in their fanbases, meanwhile Dream literally fucking encourages their weird ass shipping and shit, and it gets them even more obsessed with Dream and his group, since it feels like he's "validating" them.I don't know how it got this bad. Not even gaming youtubers with bad fanbases like Markiplier and Jacksepticeye got fanbases of literal obsessive Kpop stans (and they are, if not also Kpop stans themselves, the exact same type of people)
It's also why Skeppy's fanbase is just as shit, and closely tied with Dream's fanbase.
EDIT: Also I should mention for Dream, It's almost definitely on purpose. Notice how he had his stans go and harass the guy who uploaded the video? He keeps his "stans" around at his beck and call, to do whatever he wants. He literally acts as some Minecraft dictator. It's really fucking weird and I hope he learns a lesson about that shit not being okay.
EDIT 2: Yeah, it's definitely on purpose.