The main problem is this :

A supermarket could donate food that will expire shortly to anyone or any charity. But if that near to expiration date food makes someone sick for whatever reason, that person could sue the supermarket. Insurance companies would charge a lot to cover that risk so stores opt to throw it out rather than do the morally correct thing and donate it because it costs them money reducing profit for no return to the stockholders.

If the US would pass a law shielding companies from lawsuits related to donated food, then this could become the norm