Why is the main switch connected in series together with the rest of the circuits in the house?

It is connected so that it can disconnect the entire power grid in the house with one switch. This switch is usually integrated into a circuit breaker. No matter how one looks at it, this switch must be made to withstand (and be able to switch) great currents (in excess of 16A) at relatively high voltages (120V/230V).

Special care must be taken during the design phase, as the switch will most likely have to suffer severe arcing as well as heating, during normal operation.