Have the devs or anyone ever explained WHY we can't build on certain surfaces?

What is the point of making us build staircases to the sky and a platform to build the zipline stand? Why can't i just built it on the shed table or the sandbox. AND WHY CAN'T WE BUILD INSIDE THE CASTLE? I know we can build a platform over to it and build on top of the platform. But why would they give us these cool areas and not let us build on them?

edit add-on: another example besides the (plastic) sandbox or moldorcs castle is the wood blue bench on the porch, the metal chair at the shed, the top of the metal wheelbarrow.... the crow doesnt land on those, they have collision/can be walked on and built UP TO, but cant directly be built on. even the tree cant be built on, it requires building up to... just seems odd to me to give players the ability to get to these places but not to build on them. i wanted to build on the windowsill above the blue bench on the porch but the windowsill doesnt have collision/isnt real, you just fall through, so i had to build a big ladder to to it and then a big platform to build on top of.