How many years/generations are you willing to spend on a ship in the middle of space? Remember, Biodome didn't work. Are you going to join that prison for the off chance of your progeny occupying a land that we haven't even discovered yet?

And, before you suggest it, no, there will never be faster-than-light travel, and even relativistic travel is super unlikely.

The generation ship genre of science fiction is very interesting to me, but I've never read one that didn't seem absolutely horrifying. I don't think it is a realistic option. Especially if we aren't even capable of stabilizing our "closed system" known as Earth. A generation ship would be the same problem but 100 times more difficult.