Why does climate change generate desertification? According to IPCC:


  Climate change can be a significant driver of desertification and land
  degradation and can affect food production, thereby, influencing food
  security.
  
  Source: IPCC


My concern is if greenhouse gasses increase the temperature, there is more evaporation, so more clouds, so more rainfall.

Why does climate change generate desertification instead?
Most arid and semi-arid regions are normally far inland. Often those also have terrain features that don't allow for the moist air to reach, such as mountain ranges, etc.

So now if global temperatures increase, the evaporation in semi-arid regions increase as well, reducing the amount of water available for plants. At the same time moisture coming from the ocean still precipitates as rain before reaching those regions.