Why the president is called the head of state?

It's really self-explanatory.
A person is called a 'head' of a group when he or she leads it.
A government, country, province, state, etc is a 'state.'
In countries who have both monarchs and a president or prime minister, the monarch is not head of state. The head of state is involved in the nuts and bolts of operations, laws, changes, discussions, and decisions like war.
Google 'titular head of state.'