I can kinda see why Nintendo did this - rip off levels were a problem in Mario Maker, much harder to do that if you can't copy the level. Notice how many poor quality MM2 levels are the built-in demo levels (which you can freely start from) with a minor tweak? That would have happened to
everything.
But you're right that it's frustrating if you're starting out. Many Troll makers provide a "behind the scenes" version where you can inspect how the contraptions work e.g. adding a vine you can climb to see mechanisms "above" the level, or making something transparent or whatever.
I think trolls are the category where off-screen mechanisms are most important. In a puzzle level they're mostly an unnecessary confusion (where did that key come from? Well, off screen a Thwomp hit an ON/OFF switch and...) and in Kaizo who has time to look at everything on the screen, let alone think about things that aren't on the screen.
Of course once you're actually "Into" it there are private discords and so on for discussing stuff where people upload video, provide codes for their "Uno mas" demos (ie very short courses that show off a single new idea) and explain insights into the underlying mechanisms. I never got that deeply into MM2 but it's clear that people writing trolls for Carl or Geek are co-operating.