There are lots of possible answers. It could be that intelligent life is pretty rare, so that even if there was a species out there building dyson spheres or doing other crazy things in a far distant star system we wouldn't be able to detect them. Or maybe there is a sociological explanation, like civilizations tend to be unstable and collapse in on themselves before they can become type 1.5 civilizations. Or maybe most intelligent species are trapped in their solar systems or their home planet, that interstellar colonization will never be economically feasible.

technically I believe rogue planets orbit the galactic center

the kaizon are basically a scavenger species, they weren't actually advanced technologically, they had just stolen some space ships

probably a lot if you can build skyscrapers tall enough and they all sleep in pods.

"
He believed that the globe should unite in a massive space exploration program: one initially aimed at building human settlements on the Moon and on Mars, but then across other galaxies."

From your link. Think they might have skipped a step or two.

Anyway, I think the Earth and humanity will be just fine for tens of thousands of years to come. But we should colonize other planets and solar systems as well, because why not, and just in case.

We'll have a colony there in less than five hundred years.

It's possible, but the universe is really old, so even though we are seeing stuff that happened 10,000 years ago or 50,000 years ago, the universe is around 14 billion years old, so theoretically there should still have been alien life 50,000 years ago somewhere that we can see it. Unless alien life is just extremely, extremely rare. Or stealthy.

The thing is, based upon our best guesses about where humans will be in 10,000 years or 20,000 years (which are obviously just speculation), it only takes one alien civilization to expand throughout the stars. And within the context of the time frame of the universe, it doesn't really take that long to colonize a galaxy. And you'd think that a galactic civilization would have left some observable marks on the galaxy it conquered.

So the fact that it hasn't happened forces us to re-examine our assumptions. Like maybe we're stuck on this stupid rock for good. Or maybe civilizations inevitably collapse prior to becoming interstellar. Maybe class M planets are the only ones that can sustain life, and they are incredibly rare and highly valued and aliens fight wars of extinction over them. Maybe there is some hard to limit to how far technology can develop. Or maybe after a while species' lose their will to power and begin to decay. Or maybe a lot of alien civilizations value homeostasis over expansion. Or maybe it is possible to expand throughout the stars, but it takes a really, really, really long time. Or maybe alien life is extremely rare. There were a few civilizations in our galaxy, but they died out for whatever reason, are we are essentially alone at least for our galaxy our even maybe our local group.

who knows.

Several hundred years. Presumably we'll get a lot better at viewing exoplanets over the next hundred years, so by examining their atmospheres' we'll be able to find some juicy targets. Then it is just a matter of building the probe. I believe technology will continue to be developed at a very fast rate and 200 to 300 years from now we'll have some pretty crazy tech for space exploration capable of travelling at some modest but reasonable fraction of C.