How does cash vary in value? If you have a $50 bill, you can expect to buy a very predictable amount of food (for example) next week, regardless of what happens. Not so much if you have 0.0521 BTC.
That's true, but most vary fluctuate wildly with respect to the USD, not in their purchasing power within their country of origin, even less so in the span of weeks or days.
For example, if I was paid the equivalent of a 50 USD ride in Bitcoin in Jan 4, and were to go to buy food today, I could only buy 40 dollars worth of food.
Which currency does that? Sure there may be a few examples in case of extreme inflation, etc, but it's not the norm at all.