Because it might not fit in the tablet box.

@@kanizabbas2172you should consider that the dishwasher is programmed to receive the soap at a certain time. By putting it on the top rack from the start the tablet dissolves and is drained out with the initial rinse cycle which is designed to get the food particles off. By the time your dishwasher thinks it’s washing with soap, it’s just water. So your dishes won’t really look that great and may not be as sanitary as you would prefer. You can use a knife to cut down your homemade tablet and place any cuttings from the tablet into the detergent area and your results might improve. So unless you’re the type of person who cleans the dishes before they go into the machine (time waste in my opinion) you should figure out how to make the table at smaller. There are also KZfaq videos about how a dishwasher works that will explain this if you want to look at them. Have a great day.