Why isn't making a person as good as killing a person is bad

Possibly because...

I don't think contraception is tantamount to murder.

You have judged. It's possible that this is all there is to it... not killing people who do not want to die might just be a terminal value for humans, while creating people who would want to be created might not be a terminal value.

(Might. If you think that it's an instrumental value in favor of some other terminal goal, you should look for it)