This is true of most old empires. Romans, Persians, Arabs, Mongols, early Ottomans generally don't setttle areas they conquer. They don't engage in industrial extraction either. They form a ruling class and collect taxes. Any cultural influence that have was because of longevity rather than any interest in forced conversion. People project their impressions of more modern empires to pre-industrial ones that didn't have the same aspirations or even institutions to accomplish those aspirations.