why can the US government be compared with a tree?

The US government can be compared with a tree with a strong trunk and three branches. Where trunk is american people with their ability to vote and branches: legislative, executive and judicial. The legislative branch consists of the House of Representatives and the Senate. The executive branch consists of the president, the vice president and 15 Cabinet-level executive departments. The judicial branch consists of the Supreme Court and the lower federal courts, including the Courts of Appeal, federal district courts, bankruptcy courts, and courts of federal claims.

Примерно так: Поэтому правительство США можно сравнить с деровом.