I think your right here. The rise of the sound bar as installed below many TVs, I believe is a direct response to the preponderance of modern LCD/LED TVs to have as small a footprint as possible - the lack of depth leads to the installation of smaller speakers of perhaps lesser quality because there is no cabinet within which to place larger fuller range speakers.

People who choose to make use of a sound bar are using it to enhance/replace the speaker array that the TV came equipped with.