How is a knee joint poorly protected and vulnerable to injuries?

See below:: Knee is superficial, ie, without much muscle protecting/covering it, while other 2 are deeply located and nicely protected. The knee is easily injured for lack of the muscles; the knee, being a weight bearing joint is prone to twisting injuries, with strains/sprains frequently occurring, many times more so than the other 2. Also knee is an unstable joint with no inherent stability, like the hip.