And how do you know the proof of reserves is accurate? What's stopping an exchange taking out a loan the day before an audit and paying it back the day after? Or borrowing funds from one of their affiliated companies or entities? Or even just the CEO topping up the exchange wallets with their own funds prior to the audit. This exact thing has happened before with Bitfinex handing hundreds of millions of dollars of crypto to Tether on the very morning of their independent audits. And yet USDT continues to have a market cap of $65 billion, despite being a fractional reserve scam. Proof of reserves proves nothing.
If they've taken loans days just before the audit it's got to be noticed. If they've seen it they'll publish it. I don't know if proofs of reserve aren't accurate, there isn't a way to find out which exchanges have habits for boasting unverified figures. Falsifying market cap isn't short of fraud. USDT isn't secure if it's backed by one exchange but it's used most for bitcoin transactions.
But without proof of liabilities, proof of reserves is meaningless. So an exchange proves they have 100,000 BTC in their wallets. What if they have 200,000 BTC in liabilities? What if they have huge outstanding loan repayments or debts they hide from the auditors? How do you even know you can trust the auditors?
It's all trust upon trust upon trust. There is no independent verification.
If audits get carried out it's their auditors jobs to go through everything but I'm with you, exchanges hiding their liabilities wouldn't be a surprise. It's happened before, if they're determined to hide something they'll do it. Independent verification wouldn't translate to much, the risk's too much. Where did the owner withdraw 100,000 bitcoin to?