Explain why the chemical formulas of ionic compounds are usually the same as their empirical formulas.

All the ionic compounds are simplest combination of the atoms. All the empirical formulas are the simplest way to write the chemical formulas. Thus, the ionic compound is usually in its empirical state.  

Explanation:

Ionic compound are the chemical compound which are composed of the ions which are held together by the strong electrostatic forces which is termed as ionic bonding. The ionic compound is overall neutral, but it consists of positively charged ions which are known as cations and also negatively charged ions which are known as anions.

Since all the ionic compounds are simplest combination of the atoms and also,  all the empirical formulas are simplest way to write the chemical formulas, the ionic compound is usually in its empirical state.  

Thus it can be stated that:

All the ionic compounds are the empirical formulas, but not all the empirical formulas are the ionic compounds.