The reason is simple: "Because plants do not grow for us to eat them." The cell structure of most biological materials is very durable, and not easily broken down. Therefore it is difficult for the matter inside to be released, much less absorbed into the human body.
道理很簡單，「因為植物長出來不是給人吃的，」大部分生物體的細胞構造都非常堅實，不易被打開，裡面的物質不易釋放出來，更遑論被人體吸收利用了。