To avoid having to slow down development of new PostgreSQL features. Improving upgrades in a way where PostgreSQL does not need to either maintain multiple different versions of parts of the code and/or lock down internal interfaces which now can change freely every major version so they cannot be refactored and improved in the future is not a trivial task, maybe even impossible. Even just the existence of pg_upgrade has to some degree limited what can be done to improve PostgreSQL. Obviously pg_upgrade is worth it, but hindering development even further might not be popular.

The PostgreSQL team simply does not have the resources to do this. At least not without significantly slowing down development of everything else which there is no political will for. Maybe someone will come up with a genius idea which solves this but I am doubtful. Usually there is no free lunch.

Maybe some core dev will correct me, I am quite familiar with the PostgreSQL project but not one of the core devs. :)