Superstitions are all rooted in life much earlier, when people believed far more in portents, omens, and so on - so the time was ripe for someone to make up a superstition about anything at all - and with word of mouth, it would spread around a village, and from village to village, and then to towns.
Remember, life was hard, brutal and short, and people were far more concerned with what may happen to them through pregnancy, births, illnesses, and so on, all day-to-day occurences - al ripe for someone to invest a coincidence with 'something' which makes the originator appear to have some secret knowledge, and a person to be believed.
Some - like the famous 'ladder' one, are firmly rooted in experience. In medeval times, justice was somewhat more arbitrary, and a lynch mob may throw a ladder against a wall to hold a quick execution. Anyone heading under the ladder could have been seen as inteferring, and may well suffer the same fate - so in that case, the warning not to walk under ladders has a safety basis, which is why it is one of the most remembered, and passed on.
Of course, modern society has no need of portents and omens, so we largely ignore them, but some are passed down to people who know intellectually that there is no truth, but they still 'wonder', and pass the superstitions on.
So, it boils down to ratrional modern thought versus ancient supersitions ominous and porentious times - and we have not entirely outgrown those earlier days.
The reason is simple - everone learns superstitions from their parents or close family while growing up - people whom you subconciously trust to tell you the truth - even when they don;t, and you know they don't. Superstition thirves on fears and ignorance, and in a few generations, they will be gone completely.