(As it stands now) It is a multi-generation trip. These humans need to live somewhere and eat something, thus it's logical choice just to ship biome with them. Additional advantage is that error-prone 'boot' process to built up new biome at destination is eliminated.

But this is not the main point. It's still extremely expensive. Imagine that world GDP needs to be taken for few years just to build single ship. Who would agree to that in democratic society?