Hi @abtb256 This is because the stomach produces acid. One of the key enzymes that breaks down starch is called salivary amylase and it does not function well in acidic environments.
The digestion of complex carbohydrates such as starch starts in the mouth through the action of salivary amylase. You can verify this yourself! if you chew ugali/sima long enough, it starts to have a sweet sugary taste. This shows the enzyme is breaking down starch into simple sugars such as glucose!