Various reasons:
1) We already have the means to effectively destroy our civilization/species a nuclear war or via biological war or terrorism and the odds of that are already not zero. So in some respects that ship has already sailed.
2) If you live in a country significantly weaker than superpower category then there is already a finite chance that another culture could invade you and crush your civilization. So, again, in some respects that ship has already sailed, at least for much of our current population.
3) Because the universe is a dangerous place and sooner or later, whether in the form of an asteroid, a GRB, a supernova, colliding neutron stars, or a super-volcano eruption (Yellowstone is ticking) or something else we haven't discovered yet, our planet, and possibly our solar system is going to get hit with an event sufficient to destroy our civilization and/or our species and the only way to prevent/survive that will be to have offshoots of our species somewhere else (preferably many somewhere elses).
4) Because eventually the sun is going to go red giant, and the only way to survive that is to have some portion of our species somewhere else, and/or to have the means to move our species somewhere else (or move in a new star or re-engineer the one we have). The means to do any of that either grows out of developing interplanetary and interstellar travel or learning to manage (technically and socially) mass and energy flows that could wipe out life on Earth or allow someone to conquer it.
5) While it may currently appear that there is no one out there who could threaten us, that situation could change at any moment and until/unless we have the means to either match their abilities or escape them/spread ourselves too widely to be completely conquered or destroyed, we are sitting ducks (see paranoia is a two way street).
My 2c worth,
Todd
1) We already have the means to effectively destroy our civilization/species a nuclear war or via biological war or terrorism and the odds of that are already not zero. So in some respects that ship has already sailed.
2) If you live in a country significantly weaker than superpower category then there is already a finite chance that another culture could invade you and crush your civilization. So, again, in some respects that ship has already sailed, at least for much of our current population.
3) Because the universe is a dangerous place and sooner or later, whether in the form of an asteroid, a GRB, a supernova, colliding neutron stars, or a super-volcano eruption (Yellowstone is ticking) or something else we haven't discovered yet, our planet, and possibly our solar system is going to get hit with an event sufficient to destroy our civilization and/or our species and the only way to prevent/survive that will be to have offshoots of our species somewhere else (preferably many somewhere elses).
4) Because eventually the sun is going to go red giant, and the only way to survive that is to have some portion of our species somewhere else, and/or to have the means to move our species somewhere else (or move in a new star or re-engineer the one we have). The means to do any of that either grows out of developing interplanetary and interstellar travel or learning to manage (technically and socially) mass and energy flows that could wipe out life on Earth or allow someone to conquer it.
5) While it may currently appear that there is no one out there who could threaten us, that situation could change at any moment and until/unless we have the means to either match their abilities or escape them/spread ourselves too widely to be completely conquered or destroyed, we are sitting ducks (see paranoia is a two way street).
My 2c worth,
Todd