Do monsters really belong in horror? Sure if you were being swooped on by Mothra or had Godzilla stomping on your apartment building, it might have you issuing the odd bloodcurdling shriek, but the chances of some more mundane terror or madness occuring are greater. Monsters (the fictional kind) are fun - they're just big lumbering lummoxes and don't have the unpleasant association that any of our cavalcade of animated cadavers possess.
Plus they'd be fun to draw. Jack Kirby thought so and, indeed, one of the more noteworthy runs of monster comics were created by Lee-Kirby, with them weaving monsters into their superhero tales as well.
Between Kirby's pulse-pounding pencilling and Lee's claim to the cornball (who else would come up with a title like "ZZZutak the thing that shouldn't exist"), they all but cornered the market on fun monsters.