1. Because they were concerned Moses had been gone too long, his absence as a representative of God, left them feeling God/Moses may have abandoned them, so they created or had Aaron create a golden calf out of the jewelry these slaves had brought with them (red flag)
2. a God, as they were used to worshipping idols prior to their deliverance from bondage. Their religion had plural gods at the time
3. 40 days (red flag)
10 plagues were brought down upon Egypt, the sea was parted, and yet 40 days later they are ready to build a golden calf. Again, one needs to ask themselves this simple question. Is this story more likely true... or a fable or allegory passed down through generations.
Correct, they thought Moses may have died and they were left almost leaderless (with the exception of Aaron).
Now back to original premise you put forth which was (and I'm paraphrasing) "how could the Israelites after witnessing all those miracles suddenly abandon God and act a fool cause Moses was gone a couple extra days? Because people just don't do that."
Hence my questions. Moses was gone for 40 days not just a few. They constructed the calf as an idol to God and as you mentioned they were accustomed to many gods being worshiped in the ancient near east....especially in Egypt in which they were enslaved.
Further, Moses had not yet returned with the commandments which included the commandment to not make and worship false idols.
When people are involved in situations of uncertainty coupled with stress and anxiety they sometimes panic, they sometimes riot, they sometimes act a fool. When order or the appearance of order is stripped from people some freak out, lash out, cry out, etc..... The Israelites were God's chosen people, but they were still regular, fallible people and they made a mistake.
So despite their miraculous deliverance from captivity they panicked in that moment and erorred in judgment. It happens to people all the time when stress is high. These folks were not staying at the Ritz Carlton. They really didn't know where they were, what Moses was doing, when/if he was coming back, etc......they were scared and they made an idol to represent the God that delivered them from captivity, but they shouldn't have.