We cannot explain why charge is conserved, but we call it one of the fundamental conservation laws and use the idea anyway!
Conservation of charge is an enormously useful fact of nature. It serves as a guide in a wide variety of areas of science. We don't know why it happens. This would require a better understanding of the true nature of charge than we currently have, but that does not stop us from using the fact in a great many applications.
For example, when atoms become ions, we use this fact to determine the charge on the ion. When an atom loses one electron, it becomes an ion with a charge of +1. Since the atom was neutral before the change, the total charge of electron and ion must still add up to zero.
In solving the behaviour of electric circuits we use charge conservation to account for what is known as the "junction rule" - the fact that the total electric current that approaches any junction in a circuit must equal the total current that leaves the junction. This is a key piece of information that enables us to trace the currents that flow through the circuit.
Other applications include the generation of static charge - charge is not created, but only separated during a process like rubbing a balloon in your hair. The balloon becomes negatively charged, but your hair becomes positive, and stands out from your head as each strand repels the others.
Hopefully that is enough!