Proofs of reserves & unexplained strange movements of funds will get published if they're audited.
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.
Complete verification of solvency shouldn't be a problem to detect but audits can't stop exchanges being targeted.
But without proof of liabilities, proof of reserves is meaningless. 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 simply don't share with the auditors? How can you trust the auditors.
It's all trust upon trust upon trust. There is no independent verification.