On scale, easy to say becomes easily done. (relative use of the term easy.)
The whole point is: It's NOT unfeasible, or even undoable. But, it would require 2 things that people in power aren't willing to part with: cooperation on a grand scale, and shared resources, that will likely never be seen again.
CN: Outposting isn't out of the question! It's almost mandatory. It'd suck to be stuck on an outer-ring light-gravity outpost. We have all of the technology. We COULD go interstellar - albeit sub-light, as will everything be, until we relearn laws of physics, after we master the current set. Build Colony fleets. Populate them, people WILL go. They'll go, knowing they're going to grow and and die without ever stepping onto a natural surface again. But, they'll know that they're spreading humanity.
Why do we NOT do it?