Why does SNAT happen in POSTROUTING chain and DNAT in PREROUTING chain?

Why does SNAT(modifies source IP and/or ports) happen in nat table POSTROUTING chain, i.e after routing? And why does DNAT(modifies destination IP ant/or ports) happen in PREROUTING chain? I guess latter is because there might be multiple NICs in PC with different private networks and PC does not know how to route packet if destination IP address is still publickly routable address? However, for SNAT I can not see a reason why this couldn't take place in PREROUTING.