In both sets of movies, the giant monster is mostly viewed as a hero and ally to humanity, at least as long as humanity doesn’t try to kill them, so why the two will apparently be fighting, is unclear.
Godzilla Vs. Kong will bring together the version of the giant lizard monster from 2014’s Godzilla and the forthcoming sequel and put him up against the version of King Kong that was introduced in 2017’s Kong: Skull Island.
Ultimately, fighting is what these giant monsters do and it’s part of the reason that we like to watch them.