While Tether's bank accounts being private is a problem for auditing, even if that were removed, you'd still have to somehow "snapshot" all the bank accounts an transactions, freezing things in time so you could ensure there wasn't a shell game going on while you audited.
This just isn't feasible with a federated system where each bank has their own ledger, and asynchronously tries to align it with a bunch of other ledgers.
Blockchains overall reduce throughput compared to this model, because they enforce a single ledger. But they do this while still preserving decentralized control, resulting in a tradeoff where you lose some scalability, but also remove need for a trusted mediator(s), and now anyone can audit a snapshot of the state at their leisure.
I think OP's point was more about things which exist entirely on-chain, i.e. the idea of having stock ownership tracked directly there (rather than as a proxy for real shares tracked somewhere else).
In that frame, a better example is something like the DAI stablecoin, which is backed by assets that are on-chain. So at any block, you can audit exactly how many DAI there are, exactly which assets exist to back it, and exactly what the last reported oracle prices are for those assets.