It's because internal exchange transactions are not on-chain. When you deposit crypto to a typical exchange, it sits in a hot wallet or gets moved to cold storage. Any further transactions you make on the platform, such as exchanging currency, just shift around balances on a regular database. Only when you withdraw does it show on the chain. This is not a crypto problem.
There's no technical reason why internal accounting systems didn't post to a Blockchain based ledger.
However most companies don't build their own accounting systems, it's too much of a liability and good luck getting an auditor to sign off on financials from something homegrown.
And I not even sure if a commercial off the shelf blockchain-based accounting system exists, does one?
> There's no technical reason why internal accounting systems didn't post to a Blockchain based ledger.
I agree. Using the main chain obviously costs network fees, which for the high amount of transactions involved in running an exchange is untenable. However, there's no reason an exchange couldn't use its own public blockchain to allow independent auditing.
> And I not even sure if a commercial off the shelf blockchain-based accounting system exists, does one?
Not to my knowledge, and I know some crypto projects would love to use a product like this. Could be worth doing some market research.