The Lazy Ape Theory

Nature does not generally select for intellectual curiosity and drive. Curious animals, as a rule, are infinitely more likely to get poisoned, burned, crushed, gored, trampled, pummeled, electrocuted, bitten, eaten or otherwise killed by the shit they're investigating than they are to make a species-redefining discovery. No one can say for certain how many of our distant ancestors lit themselves on fire trying to pick up burning branches, or got their nuts bitten off by wolves while trying to offer them food, or died trying to figure out which berries were safe to eat... but I guarantee it was a whole fucking lot of them. Nature doesn't select for curiosity, it selects for survival, and survival is easier if you're a lazy cunt who just sits on the same leaf or branch or rock for most of your life eating, shitting, and fucking while ignoring the rest of the world, or some scared shitless paranoid motherfucker who just fucks off at the slightest rustle of leaves or soft breeze.

Humans, whether through divine intervention or just plain dumb luck had our curiosity and hard work rewarded just enough to keep us alive and curious and working hard (and conveniently ignoring the mountains of dead apes our little breakthroughs were costing us); and we kept on being curious and kept on getting lucky and quickly rose to become the dominant species on our planet. But since nature doesn't generally select for these traits, it makes us an anomaly. In all likelihood, what few intelligent or borderline intelligent species do manage to evolve probably plateau, and probably earlier in their development rather than later. They find a niche and stick to it, develop a rudimentary tool but go no farther with it, build a crude shelter and shrug because it's good enough, etc. Laziness wins out over drive and apathy over exploration.

The Universe is filled with species who got as far as fire, or stone, or maybe bronze and decided they just couldn't be bothered anymore.