'Ya got anything else better to do? All the malls are looking the same nowdays....

Seriously, the best reason to spread out is that if ANYTHING major happens here (plague, terrorist with cobalt bombs, nutball general with cobalt bombs, asteroid strike, burp in the Sun, etc...), the entire human race is kaput.

Spread out and the odds of a doomsday on a GALACTIC scale are far far far less....