月 in words, such as 腰 腳 腦 actually means 肉。 When 肉 becomes a root of a word, it turns out to be look like 月。They just look alike. 當「肉」變成部首時，有些字看起來就會很像「月」，比如說，腦、背、脫、臘、脹……

According to my search result. because there is meat 月 particle and moon 月 particle. for example: 腿，腰，肺，脚，肾，脏，肛，肠，脉， 胶，股，肋，肪，脓etc. they are relevant to body meat. and moon 月 particle is 明，阴，望。but most are meat 月 particle. because the characters originated from archaic 肉 particle that evolve to nowadays' 月。

Why? For simplification!