So, you can kill a person, create a new person, and raise them to be about equivalent to the original person (on average; this makes a bit more sense if we do it many times so the distribution of people, life outcomes, etc is similar). I guess your question is, why don't we do this (aside from the cost)? A few reasons come to mind:

1. It would contradict the person's preferences to die more than it contradicts the non-existing people's preferences to never exist.
2. It would cause emotional suffering to people who know the person.
3. If people knew that people were being killed in this way, they would justifiably be scared that they might be killed and work to prevent this.
4. Living in a society requires cooperating with other members of the society by obeying rules such as not killing people (even if you buy murder offsets, which is kind of like what this is). Defection (by murdering people) might temporarily satisfy your values better, but even if this is the case, the usual reasons not to defect in iterated prisoner's dilemma apply here.
5. It would require overriding people's moral heuristics against murder. This is a very strong moral heuristic, and it's not clear that you can do this without causing serious negative consequences.

Anyway, I highly doubt that you are in favor of murder offsets, so you must have your own reasons for this. Perhaps you could look at which ones apply to fetuses and which ones don't.