Because taking the most extreme course of preparation (genetic suicide) does not have as much effect on the end result as staying in the game, accepting one's own imperfection but continuing to try to be a net positive.
I don't have a strong opinion either way, but do we really have to call the decision to not have kids "genetic suicide?" I'm not the sole custodian of my genetics, it's not like I have some freak mutation that's going to save the species. We are largely genetically similar.