They aren't all bad, have you seen webflow.com? They've created a pretty good darn UI builder, responsive design as well. Doesn't use React though I think.
You are absolutely right. But I should confess builder was inspired and some ideas were taken from those builders interfaces. :-)
I bet this would seem easier to use, and have a much nicer finished product -- as well as being flexible if the user wants to (which of course is almost never the case with users of aforementioned tools).