Berkshire head office doesn't buy money market funds, they buy T-bills. They like to be sure.

Not just "sure".

1. You own your own T-bills so you can decide on your own immediately what to do with them.
2. Why pay someone else 0.03% to buy T-bills for you? 0.03% of $334B is about $100 million. Why give $100M away when you can keep it for yourself? Even if they were to pay only 0.01%, it's still over $30M!
3. It's easier to use your own T-bills as margin. If Berkshire wants to make a huge, let's say $100B purchase, they can do it easily. There's more than enough margin to do so and then eliminate it as the T-bills come due. The margin allowed for money market fund is different.
4. If you want to own T-bills, buy T-bills. Because money market funds own mostly T-bills but not all T-bills, they also own various other commercial paper, swaps, and other weird things that goose yields somewhat, sometimes.
5. When you have $334 billion in cash, you are bound to overwhelm any money market fund, so if you do indeed suddenly need $100B for a big purchase, you will surely cause some disruption.