Because the scale of space means that by necessity, humanity will cease to be a monolith. There’s simply no way for a culture that’s scattered across the Sol system and eventually multiple star systems to move as a single piece.

Instead, it’ll splinter into many groups that all undergo independent cultural evolution, each going in its own direction, many of which will be unimpeded by the others. Just by sheer numbers, eventually one (probably multiple) will find a better way forward that leaves the baggage of humanity’s past behind and allows it to become incredibly prosperous.

The chances of something like that happening on Earth seem much lower. We’re too stuck within our local maxima and too beholden to self-preserving power structures for substantial change to occur.