To make it really simple, we're a social ape with an only half-smart brain. We like being near other apes who think alike, so we make each other think the same ideas even if those ideas are crazy. And we can't do math or probability well, so we create our own crazy ideas out of thin air. And we have a bunch of bad, illogical brain behaviors that prevent us from accepting our mistakes or relearning from those mistakes or even noticing that they were mistakes in the first place.

And so, because we're all convincing each other of hogwash while being bad at math and logic with faulty brain organs, most of our species, in one way or another, believes in many different superstitious things (occasionally even contradictory superstitious things!). But at least we can take solace in the fact that we're still smart enough to reach the moon.

People do this all the time with all sorts of things, but it becomes extremely apparent when applied to the supernatural because either supernatural things simply don't exist OR they're so elusive that "evidence" only appears to people who already believe in them. Thus, people can point out other peoples' superstitions extremely easily. Because the observer doesn't hold the same superstitious beliefs, it's self-evident to them that the supernatural events in question are completely ludicrous, and because they are also filtering out contradictory events, other peoples' "obviously false" superstitious beliefs rarely ever result in the revelation that the observer's superstitious beliefs are also obviously false.

And there you have it. That's how you can get a good 6 billion people to accept several thousand different "spiritual philosophies" while constantly bickering to each other about reincarnation and resurrection and spiritual mediums and rain dances and reiki healing and evil eyes and black cats and walking under ladders and ghosts and angels and daevas and dowsing rods. They all have vested interests in remaining parts of their ideologically homogenous groups, their abilities to assess probability and causality are fundamentally flawed, and they all filter out opposing information whilst deriding "all those stupid lunatics" for accepting slightly different magical nonsense than they do.

And then, of course, there are the very rare people typically self-described as skeptics, who think literally all of it is nonsense and are so diligently introspective that they can somewhat overcome these hurdles and avoid being superstitious. But nobody likes us, because we're party poopers who write up lengthy diatribes in inappropriate situations (like, for existence, on a language-learning forum).

Let's say you are feeling unlucky (itself a superstition). So you go see a psychic, and the psychic gives you information that, when you follow it, nets you a 1000 dollar lottery ticket. You attribute this to the psychic, despite the information being something hilariously vague like "I see you taking chances in the future. You'll find a willingness to courageously follow your choices, and they will bring you great fortune." An analytical person will realize that this information is bogus, the lottery ticket is highly unlikely, and nothing in between the two events is even remotely causally linked, but many people aren't analytical.

3. But why don't you notice that the psychic was wrong when you get another lottery ticket and it fails?! Well, now we're getting into cognitive biases. These are a variety of mental shortcomings; they're essentially the logical/mental equivalents to experiential heuristics. In particular reference to the sort of behavior that encourages superstition, many people will selectively remember and amplify "winning situations" in their head and ignore or downplay "losing situations". They'll also attribute strings of unconnected events together as though they were causal chains, and they'll outright ignore information that runs counter to their currently-held beliefs. What does that mean when someone gets that first failing lottery ticket? Well, they forget it. Then they forget the next ten. But then they win another for $50, and they now connect it to the first winning ticket despite the fact that the two tickets were weeks or months apart. Then they connect both wins to that psychic. Then they lose another twenty, and when a concerned friend points out that their current win-loss ratio is 2-30, they'll completely ignore the friend and make up excuses to rationalize their superstitious belief in lottery-determining psychics.

Superstitious beliefs are beliefs in supernatural causality, which is to say that they're beliefs that magic affects the observable universe. They range from very primitive superstitious beliefs (e.g. monsters exist in the dark, and breaking mirrors brings bad luck) to organized and institutionalized superstitious beliefs (e.g. most religious beliefs, like the existence of miracles).

The reasons for such beliefs are far more varied, and I could hardly even begin to fully explain them (nor do I even think I'm particularly qualified to do so). However, the main psychological underpinnings for such thoughts are obvious enough:

1. Tribalism, the tendency to associate with an insular group of people and adjust ones' behavior and thoughts to match the group, will lead many people to accept long-standing superstitions and religions simply so that they aren't ostracized from or persecuted by their tribe.

2. The human brain creates things known as heuristics. These are essentially really hasty algorithms to help intuitively determine the likelihood of a particular event. However, because of our intuitive ignorance of actual probability, people very frequently create false heuristics. For instance, you may be in a jungle and see a flower, and when you try to pick up that flower, you get bit by a bug inside it. You create a heuristic to avoid all such flowers because they all have bugs, when the reality is that this bug was only there by coincidence and most never go near the flowers. However, because you're now avoiding the flowers, you never get the chance to readjust this heuristic to realize that the flowers are not, in fact, dangerous. This behavior majorly contributes to superstitious beliefs.

I don't believe in superstitions, because they are the cause of bad luck.