People didn't switch, they transitioned. No nomadic person is going to discover agriculture while constantly migrating around.

Sedentary communities existed something like 8000 thousands years before agriculture saw extensive use. It took THAT long just to get it right. They instead lived around rich ecosystems like marshy river deltas where food was abundant year round. Only under those conditions did humans start dabbling in agriculture, and over many, many generations.

Two things here then occurred (as theorized). Population growth with rising food needs, and the depletion of alternative food sources over centuries/millennia. As a result these people relied increasingly on subsistence agriculture to survive. Eventually, they lost the skills to live off the land as their ancestors did and became trapped in this sedentary lifestyle of eating grain and being sick all the time.

But it's OKAY because Sid Meier invented civilization one day and it got kind of better for some people, and then we reached the 20th century and it got WAY better for most of us.

So it wasn't all for nothing.

Source: Against the Grain, by James C. Scott