A fair exchange?

So what I'm asking myself why aren't there any fair or proper exchanges?

It would be easy running a proper, fair and transparent exchange, wouldn't it? Just have public known wallets and put the customer fund there. Everyone could look at them and check if the funds are there. The exchange could earn enough to survive by a percentage fee.

Couldn't you setup an exchange by a smart contract which does that? Additionally it could enable every token holder to withdraw his own tokens. Would something like that work?

Same for a stablecoin. Just take 2% of every incoming fund, take that as your earning and put the dollar amount in a bank account.

Or are there such institutions? And would you trust them if they would exist?