Part of the ipv6 ambition was fixing all the suboptimally allocated ipv4 routes. They considered your idea and decided against it for that reason. But had they done it, we would've already been on v6 for years and had plenty of time to build some cleaner routes too.

I think they also wanted to kill NAT and DHCP everywhere, so there's SLAAC by default. But turns out NAT is rather user-friendly in many cases!