The issue with the metadata, unlike in Plutus contract, is that they do not and I think cannot not keep/track the state without processing the relevant transactions, which is very computationally expensive. That is one of the other reasons why we need metadata server.
I highly recommend you guys to read the relevant research papers, starting with, UTXOma, EUTXO and EUTXOma papers. You can find them in IOHK research library.