The answer is: They didn't know what had happened to Moses

Exo 32:1

When Moses didn't return from the mountain after receiving the commandments, the Israelites found themselves in a frightening limbo. You can imagine the anxiety bubbling over in the desert camp; their leader—a man they depended on—vanished. Without Moses, their uncertainty led them to seek something concrete to ground their fears and feelings. And what felt easier than a shiny golden calf? The sight of a tangible idol was perhaps reassuring in a distressed situation, especially since throughout history, many peoples around them resorted to idol worship.

The act of making the golden calf not only reflects their fear and uncertainty, but also speaks volumes about the human tendency to gravitate towards the familiar. The Israelites, having spent many generations in Egypt, were accustomed to a pantheon of gods represented by physical forms, and in absence of guidance from Moses—the very representative voice of God—they assimilated back into these old habits.

Interestingly, in Exodus 34, when Moses eventually returns, he discovers a wholly different people than the ones he had once led out of slavery. Their rebellion serves as a striking reminder of how down-to-earth humanity truly is; we're often quick to seek out something we can see, touch, and manipulate when divine silence stretches too long. This event holds multiple echoes throughout the text of Israel's journey, particularly as they continue to battle idolatry long after leaving Egypt, showing how persistent human nature can be in seeking security through the familiar, even to their own spiritual detriment.