[candiodari]

Because it only needs one random "hit". After that self-replication makes sure it doesn't have to happen again.

Btw: the first self-replicating molecules wouldn't have been particularly complex. It may have been a half-broken piece of RNA, not from base atoms but from nucleic acids. And it only matters that it formed RNA (it does not matter what the code "in" the RNA was. It would be self-replicating no matter what that code was, so there were many valid possibilities). Also there's no need to get the "ladder" part of the molecule right. Yes it requires random chance, sure, but it may not be all that unlikely.