A savings account without a fixed term requires that the bank balances that with reasonably short instruments. If it's a huge sum they will get nervous since th

A savings account without a fixed term requires that the bank balances that with reasonably short instruments. If it's a huge sum they will get nervous since they need a certain amount of liquidity as reserves and that's expensive. Keeping liquidity over the night gives very little interest, or even negative interest in some countries.