Why we don't use this in our houses to generate the electricity? Isn't the electricity generated by the magnets strong enough? Why do we have to pay for electricity if we can just create it by pushing the magnets attached to a rotor of the generator?
You can't get 'free' energy, as energy cannot be created or destroyed. All 'free energy' devices are scams.
In order to generate electricity with a generator, you have to provide energy. If you turn the crank manually, that energy comes from your food, which comes from plants, who get their energy from the sun, which gets its energy from nuclear fusion.
If you are curious about how generators work, you can read about it here.
And to expand on Kvothe's answer, the cost of the extra food you would need would exceed the cost of the regular fuel that you save. In addition, who wants to spend their life turning a generator?