If I understand Samcheetah, he is asking about differential output amplifiers. Differential output op amps are in fact available, but they have only become available in IC form in the last few years.
Op amps have differential inputs because of the feedback configurations that become available, and because they allow one to do addition and subtraction, inversion, etc. Differential outputs are not generally needed, but are very useful for driving some of the newer A/D converters, for driving speakers, and for driving differential signal lines such as twisted pairs.
Differential amplifier designs have been available forever (almost), but true differential op amps are relatively new developments.