> The people who make these assumptions also assume, generally, that humanity will one day attain such a state. So another option is that there is some reason

> The people who make these assumptions also assume, generally, that humanity will one day attain such a state. So another option is that there is some reason it's impossible to leave the solar system. Maybe there's something just outside the solar system which we cannot prepare for and won't overcome. Perhaps any life form will run into this issue, which we may never know about, and they stay in one star system.