Why is the body organized into systems?
It works best that way.
We put labels on things to try to understand them better. "Systems" are collections of things that seem to work together for a particular outcome. Grouping things by systems makes them easier to study.
Now, WHY things are in what we perceive as systems is not really known. We conjecture that the systems are the most effective ways for some things to happen. It doesn't matter whether you subscribe to a deistic action or an evolutionary process - WE did't design the systems, so WE don't really know WHY they are the way that they are, except that they WORK.