Why does a platypus have a flat beak?

They don't.

The platypus has a leathery snout, sometimes called a bill. It is nothing like a duck's bill or a bird's beak. The platypus uses its bill to find food. It closes its eyes when underwater, and uses its bill to detect movements. Equipped with electroreceptors, the sensitive bill can sense electrical impulses, even the tiniest of movements made by underwater crustaceans.

The bill/snout is also used to shovel up the soil on the bottom of the river or creek in order to find the food. Once found, the platypus uses grinding plates in its bill, rather than teeth, to crush the food before eating it.