Because glass has property of allowing infrared radiation of short wavelength through it.but it not allow to infrared radiation with long wavelength to passes through it .
When the heat enters into the solar cooker through glass plate then it don\’t allow the escaping of heat from the solar cooker and creat green house effect
It works as collected of heat like greenhouse
So that limited amount of sunlight gets ito the cooker

The sun rays pass through the glass sheet cover and get absorbed inside the cooker box. It traps more and more heat rays of the sun, thus it is helpful in heating the food item. So solar cookers are covered with glass plate