Post GFC, regulators started assigning risk weightings to bank assets — cash and treasury bills are “riskless” by this metric, most other things aren’t. So if you need $100 of reserves, that can be $100 of treasuries or $200 of car loans or $400 of mortgage backed securities. For obvious reasons, their balance sheets are heavily invested in treasuries now.

https://www.investopedia.com/terms/t/tier-1-capital-ratio.as...