1. Maybe because Empire != Federal State
2. India was probable the oldest empire - where a roughly corresponding equivalent state is still in existence.
"The Maurya Empire was a geographically extensive and powerful empire in ancient India, ruled by the Mauryan dynasty from 321 to 185 BCE. The Empire was founded in 322 BC by Chandragupta Maurya who rapidly expanded his power westwards across central and western India, taking advantage of the disruptions of local powers in the wake of the withdrawal westward by Alexander the Great. By 320 BCE, the empire had fully occupied past northwestern India as well as defeating and conquering the satraps left by Alexander. It has been estimated that the Maurya Dynasty controlled an unprecedented one-third of the world's entire economy, was home to one-third of the world's population at the time (an estimated 50 million out of 150 million humans), contained the world's largest city of the time (Pataliputra, estimated to be larger than Rome under Emperor Trajan) and according to Megasthenes, the empire wielded a military of 600,000 infantry, 30,000 cavalry, and 9,000 war elephants."
"For the next 1500 years, India produced its classical civilizations such as the Rashtrakutas, Hoysalas and Western Gangas. During this period India is estimated to have had the largest economy of the ancient and medieval world between the 1st and 17th centuries AD, controlling between one third and one fourth of the world's wealth up to the time of the Marathas, from whence it rapidly declined during European rule."