Obviously his reason is that he wants to personally maximize his time and resources on FAI research. Because not everyone is a seed AI programmer, this reason does not apply to most everyone else. If Eliezer thinks FAI is going to probably take a few decades (which evidence seems to indicate he does), then it probably very well is in the best interest of those rationalists who aren't themselves FAI researchers to be having kids, so he wouldn't want to discourage that. (although I don't see how just explaining this would discourage anybody from having kids who you would otherwise want to.)