Wild guess: packet forward is implemented in hardware while arp/nd is software, with probably some things (think "hardware interrupt" or something alike) that allows hardware to "call" the software stack (for instance, when the link-layer addr is unknown)

So, to implement what you said, we need more than a simple router upgrade: we'd need to change the hardware, so that when a packet is forwarded from a source that's not in the mac table, the software can (asynchronously) perform an arp/nd lookup

There is probably a world of issue behind that behavior, but I do not know