Man is complex because he was created to perform many complex functions. Einstein once said, "Make everything as simple as possible ... but not simpler."
The human body is made to survive in environments from freezing cold to broiling hot. We can live on many different kinds of food. We can perform manual labor or refined intellectual contemplation. Etc etc.
Compare the human body to any machine you have ever seen. Simple machines can do one thing very well, but are generally useless for anything else. A bicycle is great for travelling on flat services, not so good on rough ground, and pretty useless for climbing mountains. An airplane goes very fast through the air, but is not so good under water. A calculator is great for performing arithmetic quickly and accurately, not so useful at telling the temperature or evaluating the creativity of a poem. Etc. But a human being can walk, climb, swim, add and subtract, distinguish hot and cold, write poems, and a million other things.
And by the way, I think you've got it backwards on complexity being a cause of medical problems. The human body can fight infection and repair itself when damaged. The ability to repair itself is awesome. I develop computer systems for a living, and just trying to build a computer system that can correct bad data or recover cleanly from a power failure is an extremely difficult problem. Usually an outside intelligence -- the programmer -- must repair damaged data, the system can't fix itself.