Apparently because the person who gave it its name likes to keep it simple. The AK-47 assault rifle – also known as the Kalashnikov – was first manufactured in 1947, hence its name.
Bonus fact 1: Its newer version, the AK-74, was produced in 1974.
Bonus fact 2: the AK, stands for Automat Kalashnikov. The latter stands for its inventor surname.
Bonus fact 3: The Kalashnikov is the most popular assault rifle in the world, with about 100 million units manufactured. Being a very cheap and easy-to-operate assault rifle certainly helped.
Bonus Fact 4: The Kalashnikov is one of the most reliable and durable rifles ever made. It will shoot anytime anywhere.
Bonus fact 5: When it was first developed in the 1940’s it was lighter and shorter than the period’s other assault rifles. But today its major drawback is its heavy weight. It is also considered not very accurate and suffers from incompatibility with modern Western accessories, as it was not designed to fit.