Why were philosophers from times long gone so fixated on God?

Not necessarily God, but the concept of a supreme ruler/creator. Many of the big names were obsessed over refuting or solidifying the existence of a ruler, which ruined a lot of cool philosophy imho. Kant for example, why spend so much time on the possibility of God when you have so many more things to say on humanistic philosophy? Why can’t we just drop the notion of God and discuss other more human concepts?