Lets not forget the other reason that no one is mentioning.

A) Statistically speaking it is highly doubtful that we are the only sentient species in the small region of space within our own galaxy.

B) Theory of relativity aside it is also statistically imporbable that most other setient species wont one day decide to expand on their own and put their eggs in more than one basket.

C) The species that develops the technology to do this before the others will have a great advantage over the species that stays planet bound and squanders their rescources on hedonist pursuits over interplanetary expansion.

D) Based upon the known historical evidence here on our own planet, there is nothing to sugest that evolutionary expansion between stars is nessesarally going to be any more peaceful that evolutionaray expansion was here on earth between different regions.

Do you want to be the spannish conquistadors or the mayans in the above enevitable scenario?

It is simple common sence to expand off planet as soon as possible.