The vast majority of bank accounts offer abysmal interest rates. And holding a stable coin is nothing like a bank deposit its like holding cash in your wallet, you and only you have control over the coin. And there are opportunities to "stake" the coin and earn an interest rate.
The primary reason for delays between buying crypto and actually receiving it is your bank. Not the exchange. Your bank is fucking around with your money and not sending it to the exchange immediately and even after they send it they have the ability to reverse the transaction, so the exchange needs the guarantee the money will not be taken from them.
The rest of your comment is just nonsense. Tether is a cryptocurrency I don't even understand how you could argue its not. You should look into all the laws traditional banks break using your money too.