Because the powers that the miners have are a lot less than the powers that the banks have. Miners, even at 99% hashrate, are unable to change the rules of the network. The most they can do is censor transactions and spend their own money multiple times. But even a miner spending their own money multiple times costs money - you have to redo blocks (costing tens of thousands each).
Banks on the other hand can print money, change the interest rate, steal, block people from the system for arbitrary reasons.
Most people don't realize that a 51% attack is not a 'we get to do anything we want now' attack. Miners still need to follow the fundamental rules of the network, and they also need to blow huge amounts of cash on electricity to maintain the attack. Today that would cost millions of dollars -per day-. That money needs to come from somewhere, and generally that comes from selling the coins that the miners get paid. If they make the system useless, the coins they get won't have enough value to pay the bills.
So there are a lot of dimensions that mean a cartel of miners could reasonably be construed as an acceptable alternative to banks.
Banks on the other hand can print money, change the interest rate, steal, block people from the system for arbitrary reasons.
Most people don't realize that a 51% attack is not a 'we get to do anything we want now' attack. Miners still need to follow the fundamental rules of the network, and they also need to blow huge amounts of cash on electricity to maintain the attack. Today that would cost millions of dollars -per day-. That money needs to come from somewhere, and generally that comes from selling the coins that the miners get paid. If they make the system useless, the coins they get won't have enough value to pay the bills.
So there are a lot of dimensions that mean a cartel of miners could reasonably be construed as an acceptable alternative to banks.