Adding to what others said, too little insulin can be dangerous, so if users don't know/notice that it stopped delivering more insulin, they might not manually

Adding to what others said, too little insulin can be dangerous, so if users don't know/notice that it stopped delivering more insulin, they might not manually add the requisite amounts. Forgetting to take medication is dangerous, overdosing medication is dangerous. It is just not that easy to design a system like this.