I'll turn that around: if as a bank your risk modeling does not account for outlier events, you're dead. Because the way banks and bank accounts are used these days (eg. payroll providers), the society is not willing to accept the second-order effects. As OP's example, a payroll provider was using SVB as their bank to hold funds in transit in order to make payments to third parties. Result?

    Every regulator sees the world through a lens that was painstakingly crafted over decades. The FDIC institutionally looks at this fact pattern and sees this as a single depositor over the insured deposit limit. It does not see 300,000 bounced paychecks.

But back to your point. Banks are over collateralised. Let's take an example from just this week. Credit Suisse is in trouble - their collateral has gone from ~170% to ~150% - they have the collateral to cover their total deposits, but they don't have nearly enough of them as liquid assets to cover the increased outflows.

Until last week I didn't know about HTM ("hold to maturity") and AFS ("available for sale") categories. I have since been educated.