here is a simple answer.
We humans depend on organism that either make their own food or has food. When we eat it, our body needs to break down the food to is basic component ei fat, carbohydrates, minerals, and proteins. Thats the only way we can absord these basic component into our system so we could use it (other than via intravenous of course).
Plants on the other hand, only rely on light, some gases, and basic minerasl (note that these components are simple enough to be absorb thru their roots and leaves. Once these are provide to the plant, with its clhroplast it can easily make its own food - glucose.