The 16:9 ratio is the standard set for HDTV after an international debate among professionals in the industry; I believe the final standard was promulgated by the Society of Motion Picture and Television Engineers ("SMPTE"), but if I'm wrong about that, someone will correct me.
A key factor underlying the decision was the recognition that films come in many different shapes, so that no matter what standard was chosen, one size could not fit all.
I'm new to HT and I had thought a 16x9 would be a fullscreen experience with no bars.
A misconception so common that it has its own chapter in our Beginner's Primer and FAQ. Use the link in my signature.