Penguins actually have knees
The knees were there all along — hidden under the feathers.
1Penguins waddle and look like all thigh and no joint — yet they have a full set of knees, hidden under feathers and fat, with femurs so short the whole leg looks like one piece.
2Their leg bones are the same as other birds': femur, tibia and a proper knee joint, just shortened and tucked inside the body. That is exactly why penguins waddle — short legs put the centre of gravity low, and the side-to-side stride is the result.
3The waddle is not wasteful either: studies show that although penguins walk slowly, they recover energy surprisingly well with each step. And in the water they skip legs entirely — they fly, with their wings.
Penguin knees exist — they are just short and hidden under feathers.