)
It's less about "developing space travel" and more "developing space travel that can go outside your star system". What are you gonna power a generation/sleeper ship with? Both fission and fusion last decades (in no small part because of neutron degradation of materials), not _*centuries_* needed, and everything else has way too low energy density. So far, the only feasible way to have interstellar travel (without digitality shenanigans) is _*turning your own star into propulsion system_*.