It's a bit more nuanced than that and they have, at great expense. Human subsistence strategies tend to be well optimized for the conditions they exist in. Greater Mongolia, both the modern country and what's now the province of Inner Mongolia in China, does have areas of arable land. One of the major issues is these areas tend to shift over time. Some years, one valley gets sufficient rainfall, some years, another. Pastoral communities can easily adapt to this and other marginal conditions much more effectively.

But you can also approach these issues with say, a couple centuries of large-scale agricultural development projects like China has done in Inner Mongolia. If you build massive irrigation networks and invest heavily in other technologies, you can develop the area for sedentary agriculture at the expense of the grasslands.

Farming is a _lot_ more work than hunting and gathering (if food is plenty) and people mostly resorted to it out of desperation, and farmers had to be _forced_ through slavery to do the work throughout most of history.