I don't get what's so special about CoD that Americans just buy it by default
There are lots of franchises that people buy "by default". Final Fantasy is nowhere nearly as good as it used to be, and yet it still sells many millions of copies every time a new game comes out. The brand has a certain install base that shows up at every release. You see that with most major brands (Pokemon, FIFA, etc...).
In the case of Call of Duty, the series really exploded in popularity in Generation 7. The most popular system in the US at the time was XBox360. What were the big games to play on that system? Halo, Call of Duty, etc... basically online shooters. That was what the XBox360 really excelled at and Call of Duty was the most popular one. It has that huge install base. Even if some of those people bought PS4's later, the game still has the install base that was built during the XBox360.