Because it’s a silly premise that lends itself better to light and fun as opposed to heavy and dark? Because the moment you take this seriously in a non-ironic fashion it’s an automatic L?
What is light and fun about a guy who was dragged into being the kid sidekick of a crazy vigilante 5 minutes after his parents were murdered?
Or a teenage girl with pretty much Satan for a father?
Or a runaway alien slave girl trying to stay away from her intergalactic slave masters and murderous sister?
The only light and fun character is Beast Boy and that is only because the stories about him being all depressed and sad happened back in the 1960s and all modern appearances had writers making him the happy go lucky guy who embraces his freakishness.
And aside from Teen Titans Go! the Titans as a property has never been light and fun. There have been light and fun moments but overall the Titans have been dark and depressing as fukk. Even the Teen Titans cartoon wasn't light and fun. It was mostly serious with light touches.