Greater share of wallet, greater share of habit than investing services (and therefore keeps your AUM sticky), and great monetization options on both stocks and flows of cash*.
* Not physical cash; distinguishing it from value in investments, some of which are cashlike.
So one investment option is called a “money market fund” and it is functionally not an investment, because they are (de facto) backed up by issuer and government guarantees which make them approximately as solid as insured bank deposits.