Obviously, because of history of Internet deriving from Arpanet. The whole domain name structure grew out of the needs of the US government, even if the .com domain was largest TLD from the start.

Nope, .gov belongs to the US, so they get to hog it.

It's a historical vestige, the Internet started out as a U.S. government-sponsored research network, so they built it for their own needs. There's absolutely no reason to them to give that up.

It's a perk from building the internet. Early bird gets the worm etc

In addition to all the siblings, government isn't always spelled with 'gov' so it would be useful to the subset of countries where those letters make sense. Compared to say Mexico with http://gob.mx .

Because they came up with it.