since On is used to refer to we/ people etc, why does it make use of the subject verb conjugation of il/elle. eg il est/ on est. Instead of On sommes

Technically on is the equivalent on "one" meaning one hypothetical person. In English, we also conjugate this in the third person singular. "One is" not "One are" even though "One is responsible for ones own life" can very well mean "We (people) are responsible for our own lives). 

Of course on is also used in ways one cannot be used, to mean we. My guess (and this is just that, a guess) is that the plural usage of on came into use long after its conjugation in third person singular was established and people just kept saying "On est" even in cases where it means "we". It is indeed interesting to think about