Why does the platypus have beak but it is not considered as a bird?

That is because the platybus has many mammalian features, such as the mammalian middle ear, which has two of the bones found in the jaws of birds and reptiles. The platypus does not excrete uric acid, but all birds do. The platypus has hair and mammary glands and it excretes urea, but no bird has any of these features. Platypus also lacks feathers or air sac breathing, unlike birds. Platypus has a diaphragm dividing its abdominal cavity just like other mammals, but no bird has it. Therefore either the platypus evolved all these mammalian features independently of the mammals, and at the same time lost many avian features, or it evolved the beak independently of birds. Most reasonable people, who are not barbarians, would conclude that the platypus is a mammal but not a bird.