the big players are funds & institutional investors. It's very difficult to put $100m into a cash account. Most banks want collateral and take time to come up with transfers. It's not like going to your local bank and taking out $100. If it's a mutual fund, they want the liquidity. Most funds keep a MM position that they can liquidate when necessary. If you read the prospectus of any index fund you own, most likely they are keeping a MM position. So when you have $1 billion, a .01% MM is a great way to park money instead of having a liquidity crisis on your hands due to market conditions.
Also, companies don't have cash accounts they just access. Microsoft & apple hold the majority of their cash overseas (us government would tax it if they repatriated it). They put this money to use in secure Tbills or short-term liquid assets. When you see cash on a fortune 500 balance sheet, it's rarely just cash.