Because there were much fewer bison than there now are cows, and because the atmosphere wasn't already being pumped full of CO2, methane etc. by humans.
That is incorrect. The plains bison herds expanded to match the carrying capacity of the grazing.
Now, there are a lot of other cattle that aren't being grazed on the historical prairie. You might have a point with those, but it's not relevant to the point under discussion.
