They're edible with your teeth when young/fresh....

The quality of grains that made them staples is that they could be dried/hardened. That way they keep all winter, can be traded, etc.

For an everyday example, think of beans. We sometimes eat them fresh/soft as vegetables (green beans or edamame). More often they're dried, stored, traded on commodity markets and then re-softened to be eaten as soy cheese or chili con carne.

I'm confused. The uncooked rice I buy in the store is harder than the wheat grains I occasionally eat from a field. (It takes a while to chew and makes a glue-like substance that is quite tasty when you're hungry.) Is this a new development?

Hard grains presumably are protected against bugs and so could be grown/stored without being eaten (as much) by other creatures - giving the advantage to tool users who can smash them open.