We already have banks, we already have currency,
Because banks suck and do NOT provide a good solution for all use cases where you want to hold and exchange value online. Consider stablecoins and crypto as a type of competitor to some banking services for some uses.
Banks take 24 hours to settle your transfer and charge you a 5% fee for the privilege. Instead of the minutes or at most an hour you would possibly want your transfers to complete in.
Banks will create waiting periods and holding periods to abuse the float at their profit and your expense.
Banks will randomly hold or freeze funds or transactions for review.
Banks cancel your transfer, because they saw something in the Memo field that tripped an alert. The next day police are at your door grilling you about your typo'd Memo line that auto-corrected to hookers when you meant to type hooks.
Banks take back the money from the digital thing you sold online AND charge you an exorbitant fee per transaction for the privilege, because the buyer was a scammer who decided report to Paypal or the Credit card as never delivered long after they received AND used up the item.
Banks won't open a new account for you, because the teller racially profiled you at the gate.
Banks when presented with your paycheck to simply deposit will stare and the back of the check for minutes and reply with "Now how did you get that? This signature must be fake.". "I'm going to shred this, and call the police to take this guy away."
Banks refuse your transaction and will not tell you why, because they saw you walking in wearing orange socks, and orange socks are deemed high risk on Wednesdays. Tomorrow the high risk colors are pink and blue.
Banks decline your request to withdraw $1k of your own money at the teller window in cash, because they Insist on knowing exactly from whom and what you are buying with $1k, and are not adequately satisfied with your explanation.
Banks close your merchant account because Mastercard received a complaint and found digital art on your website they decided is not brand-friendly.