> (along with the cost of the money you used to pay for them, which is very cheap for most large banks)
Why should depositors leave more money than they need for short-term working capital in the SVB at 0% when a Treasury bill pays 4% with less risk? When all rates were roughly zero, it was easy to just lazily leave everything in the bank; but when rates increased, the reward for leaving and the risk of staying (because the bank is now mark-to-market insolvent, and thus particularly vulnerable to a run) both increased.
When interest rates increased, the NPV of the SVB's assets decreased. Someone had to take that loss. Their managers and shareholders presumably hoped that would be the depositors, by leaving their money in a risky bank earning interest below the risk-free rate, slowly accepting the loss over time. The depositors had no economic incentive to do that though, and they didn't.
For emphasis, the SVB's problem isn't that their bonds are trading at irrationally low prices due to liquidity problems (i.e., a "fire sale"). The market price for their bonds has behaved exactly like a textbook NPV model, and similar assets trade with normal tight spreads; the price is just lower than they wanted. Hold-to-maturity accounting allowed them to ignore that for regulatory purposes, but accounting doesn't change reality.