It's the same reason I don't have a nice suit of plate armor today: it's wildly expensive. Not so much the materials, but the manufacture.
In the modern world, I don't have a helicopter. It's wildly expensive.
In fantasy magic times - the only people who fly around on brooms or carpets are the very specialists who know how to manufacture and maintain them. Random mundanes don't have the skills/training/talent to fly them, much less make one, unless you're the scrappy protagonist.