I really don't think they are. Or rather, they suggest such ONLY if the only thing you know about shows like SU and Voltron is "they're aimed at children".
Aim is one thing, actual presentation and intention is quite another. I can't speak for Voltron, but I know Steven Universe pretty well, and just because it's covered in goofy smiles and stars and musical numbers doesn't mean it's not ALSO a story containing serious themes about war, identity, and emotional development.
I doubt the seven-year-olds that are watching Steven Universe Future can even comprehend the kind of psychological turmoil that Steven is clearly going through right now. They probably just see a funny cactus going on a rampage...but that doesn't mean that's all there is to it. A show shouldn't be defined by its lowest common denominator.
I think it's also worth pointing out that a lot of shows that are "for kids" in name only end up that way because the gray area between Paw Patrol and Bojack Horseman doesn't really exist in American culture yet.
If you want to market your cartoon, you can either smother it in edgy "adult humor" and try to find a place in a very limited niche (that's almost completely dominated by comedy/slice of life, by the way)...or you can play it safer, swap the word "kill" for "destroy" in your script, and try to catch Disney Channel's interest.
Basically, you either have to do your best to make your content accessible to kindergartners, or you have to swing all the way to the other extreme and make it adults only. There's very little room for anything in between...so I don't think the fact that an American cartoon is labeled "children 7+" is actually a reliable description of its content. I mean, if both Gumball and Avatar TLA can conceivably fit into that label, how much is it really worth??
TL;DR, before you assume that the cartoons listed as examples in this thread are just "too childish" to have people asking reasonable questions about their quality, you may want to consider the fact that many of them would qualify as seinen anime if they were made in Japan. =/ Don't judge a book by its cover.