It's just a fundamental law of physics.
You may well ask why is the speed of light what it is?
Why is Planks constant what it is?
If you can answer those kind of questions then you'd be a lot more knowledgable than a lot of Quantum Phyisists.
We just know that some things ARE. We don't necessarily know the WHY of everything though.
It's related to the fact that the relationship between current, magnetic field and force in Flemings laws is dependent on whether current is applied directly to a coil (motor principle), or inducted into a coil (generator principle). Self inductance is the coil trying to act on both Fleming's left hand AND right hand, rules at the same time.
You can see this if you hold both hands up in the left hand and right hand rule position. Motion (the direction of change in magnitude of the current) is common. Current direction is common. Field direction is opposite.