First, you are not getting money market yield (worst in case of 0% checking account and very suboptimal in case of saving account).
Second, bank will lend out your money to people around you that buy scarce assets (eg real estate) that you might be saving for and make them more expensive for you (pumping price with your very own cash).
I find it fascinating that many people, even if they more or less understand fiat system don’t realise the extent to which it is unfair.