Well, for starters, zero seems like an important point, as in if it's risk free then it's qualitatively different from something that has risk. Even govt bonds

Well, for starters, zero seems like an important point, as in if it's risk free then it's qualitatively different from something that has risk. Even govt bonds have risk because although they have a predefined payout, their market price can fluctuate, sometimes a lot. But MMFs --- negligible risk.