Probably for the best, since it sounds like that could be used for DDoS amplification and/or reflection.
For example, if an attack could spoof traffic to get two different reflectors hall-of-mirror-ing each other, or using a botnet that spoofs traffic to get one collection of dupes to slam a single victim in response, etc.
How would you spoof multiple valid packets in a TCP-based protocol requiring a sequence of interactions when you can't receive any of the ACKs (because they'll be sent to not-your-IP)?
For example, if an attack could spoof traffic to get two different reflectors hall-of-mirror-ing each other, or using a botnet that spoofs traffic to get one collection of dupes to slam a single victim in response, etc.