Why there is no driver circuit needed between 8051 microcontroller and an optoisolater?

Are you referring to the original Intel 8051, or one of the many variants? The 8051 has weak internal pullups on the i/o pins, and can source only about 60 ua, but can sink 1.6 ma, still not much when it comes to driving the led in an opto. Some 8051-based dervatives can sink much more current. Atmel's 89C2051 for instance, can sink up to 20 ma per i/o pin. This can directly drive most optos.