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.

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

No, I do mean the ripe ones. I am a bit into survival food and know the differences - but as I said, only of the grains I tried so far. I could chew them with no big problems.

With the rest you said, I agree ...

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?

The uncooked rice you buy in the store has been dried.

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.