why magnetic monopoles does not exist?
Answers
The fundamental reason is, nature isn’t determined by what seems reasonable to us.
It sounds reasonable to us that there should be magnetic monopoles. It also seems reasonable that there should be two signs of gravitation, like there are two signs of electric charge. And that electrons in atoms should really be point charges. And that electromagnetic waves should propagate through something (call it ether).
And sometimes what seems reasonable turns out to be right, like some symmetry principles in physics. But not everything. And we don’t know in advance.
The fundamental problem with what seems reasonable is, it’s based on our experience somewhere else. And somewhere else can be different.
The right way to use our expectations (another way of saying, what seems reasonable) is to test them. Make predictions based on our expectations and then test those predictions. And, when the predictions turn out to be wrong, recognize that this new place is different from the old place we know. And learn about the new place. And expand the range of what seems reasonable to us.