Oh, YA Guy knows they weren't only about monsters. I wrote a whole book about the other things they were about. Pick up a copy if you're in the mood.
But in classic films such as King Kong (1933) and Godzilla (1954), as well as modern classics like Alien (1979) and The Thing (1982), the driving force of the movie was the encounter with and--usually--defeat of the monster.
That was then, this is now.
These days, the driving force of monster movies is . . . bad family melodrama.
It began in the nineties--which, not coincidentally, was the decade that the "family values" campaign picked up steam in popular and political culture. The rhetoric of that campaign was complex and not very coherent, but it boiled down to the following: traditional (i.e., male-headed) families were in crisis, with an ensuing social breakdown epitomized by the out-of-control breeding of marginal (i.e., black and Hispanic) females; therefore, traditional (i.e., white) men needed to reassert control over their families and their society.
This discourse drives all three Jurassic Park movies, wherein nice-guy white male scientists have to save kids from ferocious, out-of-control female dinosaurs. It drives War of the Worlds (the Tom Cruise version), wherein a down-and-out dad becomes Father of the Year by fighting off dark-skinned, pregnant-bellied alien tripods. It drives Godzilla (the Roland Emmerich version), wherein a nice-guy white male scientist regains his girl (and thus presumably starts down the road to establishing a traditional family) by fighting off pregnant monster-lizards. It drives every movie Roland Emmerich ever made, but that's perhaps beside the point.
And now along comes Godzilla (2014), an unholy mess of a movie in which the title creature appears forty-five minutes into the film and occupies roughly five minutes of screen time thereafter, if you add up all the five-second segments of him slugging it out with giant pregnant insectoid thingamajigs. The rest of this painfully bad movie is taken up with the following: the story of a young army weapons expert, abandoned by his cuckoo conspiracy-theory father, who then abandons his own wife and young child when he's called to spring his nut-job dad from jail on the eve of the giant creatures' rampage. He reconciles with his dad (what a good son!), saves a little Japanese boy from the monsters (what a good father!), implausibly survives a thousand-foot fall from a train track so he can destroy all the monster-bug's offspring (what a really good father!), and even more implausibly, reunites with his wife and son, who have somehow avoided being crushed by the city that basically collapsed on them (what a really, really good father!).
Godzilla and the other monsters are entirely incidental to all this. But in Hollywood, where rich white men make most of the movies and most of the decisions, it's apparently really, really, really important to keep reasserting how really, really, really screwed up society would be if white fathers weren't in charge. So they dream up bigger and bigger CGI monsters to threaten society, only so white fathers can save it in more and more preposterous ways.
It's really getting to be a drag. I go to monster movies to see monsters, not to see rich white guys live out their anxious fantasies about social control.
I guess I need to go back to 1954, when Godzilla--and not some anxious rich white guy--was truly the King of Monsters.