Please Explain

Can someone please explain to me how the devise I depend on to clean my families clothes, that I use with soap and water at least 2 times a day, can be FILTHY inside. How do the clothes get clean when the washing machine is so dirty inside??? Have I been sending my sweet family out in dirty clothes and just didn't know it? Has anyone invented a "self-cleaning" washing machine? Seems kinda silly doesn't it.