Because if they are connected in series then if one of them is turned off the rest will also shut off and also because in series connection the potential is distributed while for a typical household appliance the voltage should be the same.
Why Are Household Appliances Connected In Parallel?
If connected in series, the same current passes through all the household appliances. Also, if any one appliance malfunctions, it prevents current from flowing to the other appliances and thus they also don't work. So in most of the cases , household appliances are connected parallel.
According to some electrical laws we will not connected all heavy appliances in series. And If connected in series, current passes through all the household appliances at same time which are not functioning.
Household appliances need a have a certain voltage to work properly, but their current needs differ considerably.
A parallel circuit allows each appliance to access the same voltage when it is plugged into any outlet in the circuit. It then can draw the amount of current it needs to run properly.
The circuit is protected by a "circuit breaker" in the electrical panel which shuts the circuit off if the total amount of current drawn by all the appliances in the circuit exceeds the capacity of the circuit to function safely.
In a series circuit, the first appliance causes a "voltage drop." That means the voltage available for the next appliance that is "plugged in" does not have the same voltage available to it as the first appliance and it will not work properly if at all. (Voltage is like a "push," and the amount of "push" declines after each appliance gets its "push.")