For one, it doesn't help that Steven Universe promotes some of the manchild behavior and a bunch of Tumblr social justice stuff added in there just to spice it up. You got a fat kid that has a family of mostly women taking care of him; I feel that a few people reflect themselves as the main character, and odds are you are going to have a few of them that never quite outgrew their childhood, and thus try to "return" to cartoons such as these. Also, people will fight tooth and nail for things they hold quite close to them-- hence why fanboying/fangirling flame wars are most of the times likened to a spergout, because they illogical fight against all that oppose their cartoon "Fantasty" of having crystal gems saving the world, whilst eating unhealthy food and having magical powers.
As for Sonic, you got me: probably because the Sonic Team is run by a bunch of autists these days.