Well, maybe the Greek pantheon didn't phase out? Or any of the other religions? Maybe they're still prevalent in the modern world like Christianity and Islam and Judaism? And while people believe in the Old Religions, there's very little evidence of monsters being real to the mortal world because since people stopped believing the Gods were angry and so they "turned away" from them. At least, some of them.
Many of the lesser gods still do their thing, hang out with mortals, and exist. Play their parts and take interest in certain events, and maybe Zeus is deciding it's time that they took back that power? Or one of the Gods is trying to usurp power by unleashing the monsters of old?