Source IP address field in RREP on DSR routing

Hello
I have a question about routing in MANET using Dynamic Source Routing protocol.
IN RFC4728 (DSR) in section "IP fields" of RREP (Route Reply) packet we have this:

ok.
I read in several books and also in rfc4728 that: when a source node (node that initiate route discovery process) receive a RREP it are not able to understad which node sent that RREP!!
But why?

rfc say that the value of source IP address field of RREP packet is IP address of node that send RREP. and this packet forwarded toward Source node.
My question is why??! why source node can not always understand that which node is sender of RREP?

Please guide me
tnx

I think Route Discovery is the normal routing when no source routing is present, but the whole point of source routing is to diverth the packet to a different route. As such, Source Routing is never used (ignored) in most situations, as hackers love it, use it to impersonate other hosts with a duplicate IP.