The problem is that there aren't any other planets. Venus is an inferno, Mars may as well be made of asbestos dust with a side of radiation and drought, those are the most hospitable ones in this solar system and we're not getting anywhere further anytime soon[1]. And the amount of rockets and environmental damage to launch billions of people and enough spaceships for billions of people is ... a lot.

[1] http://www.antipope.org/charlie/blog-static/2007/06/the_high...