In addition to the good points that have already been made, I would cite the following highly technical and abstruse factor: Traditions die hard :-)
Related to that, I'd imagine that a factor in many cases is that designers tend to use approaches they are familiar with, and that build upon their previous work, unless there is a compelling reason to change. And the fact that any given design is likely to be used in many systems in conjunction with associated components that are single-ended would seem to make the case for change less compelling.
Finally, making a design fully balanced adds complexity, and with it presumably more opportunity to go wrong, and more opportunity for the design and development process to become more costly and lengthy than desired. Especially if the designer has not previously used fully balanced approaches.
Best regards,
-- Al
Related to that, I'd imagine that a factor in many cases is that designers tend to use approaches they are familiar with, and that build upon their previous work, unless there is a compelling reason to change. And the fact that any given design is likely to be used in many systems in conjunction with associated components that are single-ended would seem to make the case for change less compelling.
Finally, making a design fully balanced adds complexity, and with it presumably more opportunity to go wrong, and more opportunity for the design and development process to become more costly and lengthy than desired. Especially if the designer has not previously used fully balanced approaches.
Best regards,
-- Al