Alternatively, why expand when you're introducing best case 100- year physical delays and multi-year informational delays between fragments of your society? You can't do anything with the resources at the other end that will benefit anyone at the origination. Sure, you get species redundancy, but like I said before that's just warm fuzzies. Maybe the aliens all decide to spend the effort fixing problems where they already are.
If you're at the stage of being able to pull of interstellar travel, then you're at the stage to exploit resources in your own solar system outside Earth. That would take an extremely long period of time. Sure, technology could advance to make that deplete faster, but even that type of categorization is short sighted. Kind of like back in decades past they thought we would use up all our oil by now and not taking into account more efficient usage of oil (such as cars that get more than 8 mph than the shitty boats they used to have), newly discovered oil reserves, and novel ways of extracting oil that weren't an option in the past.
The resource push outside the solar system might need to eventually occur, but I think it is more than reasonable to think it would take thousands of years to deplete the solar system completely, unless your system was virtually empty of other moons/planets. Short of discovering "unobtanium" like in Avatar, there would be little drive for resource purposes.
Your neighbors are assholes is a good reason, at least for humans.
More to the point when you look at life here on Earth its spread and established itself in every possible region it has the ability to. If we discover a seemingly habitable world around Tau Ceti why wouldnt we go expand there? Just look at humans in centuries past, lots and lots of people left their homeland to colonize some other land with no inclination of ever returning to their homeland.
If a habitable planet is relatively nearby, I absolutely believe that humans would eventually want to travel there. But that is one planet that can be reached in a "reasonable" amount of time. If there is nothing else nearby, then people aren't going to just bolt off into the cosmos to roll the dice. Without FTL, the costs and risks of traveling far out begin to exponentially increase. It might take hundreds or thousands of years even at near light speed.
Plus the other thing to consider is even if a planet has life, it still doesn't mean we can easily survive there. The organic materials there might be toxic to us, the air composition different so that we can't breath. You'd be surrounded by life that you're essentially allergic to and air you can't breath without assistance. I did like how The Expanse touched on this. Eventually they invented some machine that could solve the organics problem, but you're still at the complete mercy of your technology unless you start to genetically engineer your humans to no longer be human and be more akin to the alien life on the chosen planet.
phys.org
