From time to time people claim to have invented a machine that will run forever without energy input and develops more energy that it uses Why would you have reason to question such a machine?

Answer 1

Friction and entropy overcome all devices eventually. DaVinci called perpetual motion a shimia. A ghost or illusion.

Answer2. Such a machine would violate Conservation of Energy, creating energy from nothing.
The Second Law of Thermodynamics.

Answer 2

A machine that creates more energy than it consumes goes against the law of conservation of energy. This law states that energy cannot be created or destroyed, only transferred or converted. Therefore, claims of perpetual motion machines violate this fundamental principle of physics and are not supported by scientific evidence.