The difference is that you have to find a buyer for your switch if you want to eat, while the supermarket is required to take your cash.

This becomes even more difficult for owner large amounts of shares in the same company since you selling could have a negative effect on the shares. The reason they have the value could be that the market thinks you are in it for a long time and will provide additional value, once you start selling it could trigger a larger sell-off.

Or there could be a financial crash and the value could be cut in half.

So cash and cash-equivalent assets are special and worth more if you need to eat right now.