PoW is not self-referential. If someone shows you three blockchains, say pow1, pow2, pow3 you can roughly calculate the amount of real-world resources that were required to create each chain based on the hash difficulty. That work/effort cannot be faked. If the difficulty is high enough, you can be certain that this chain was not just generated in someone's basement last night.

With PoS, if someone shows you pos1, pos2, pos3, you have no idea which to trust without external information/trust. They all could have been generated the night before in someone's basement.