Too bad their digestive tracts can’t process grass well enough to extract any meaningful amounts of nutrition from it.
It would be like trying to get your kitty cat to live on grass (hint, cats are obligate carnivores, they will starve on a vegetarian diet).
Oh, and grasses didn’t emerge until after the K-T boundary...
Well, I never said they lasted very long that way.
It is like the guy who kept coming into the country store to buy chicks every week and, when asked why so many, replied "They won't grow! I am either burying them to deep or too close together."